Yes. This. All of this. I am not alone!:: deep breath :: Reading this is bringing me back to my teens. It's funny because I've acted similarly towards people I actually really liked when I was younger, and I'll never know if they felt towards me as you feel towards him. I'll tell you what I think.
We HATE assumptions. Period. Most of the time, we simply CAN'T make them. This is ESPECIALLY true when it comes to matters of the heart, although it is also true across the entire spectrum of our experience. It is extremely hard for us to declare a single thing "beyond the shadow of doubt," because we tend to see all sides of an argument and the possibilities therein. We can come up with a million and one reasons why something might NOT be true. (Ti + Ne)
Couple this with a general lack of sufficient "feels" to successfully navigate social or romantic situations, and you essentially end up with a person who is flying blind in matters of the heart, and not only that, we are AWARE we flying blind. We are AWARE we aren't doing very well when it comes to understanding the other person, because (especially when we are younger) we rely on logical causation and clear, articulate communication to come to conclusions; and even THEN we tend to find unexpressed variables in the causational equations of life. Even THEN we can find reasons to "doubt." This is a pretty much universal part of the INTP growth process; doubting everything we possibly can until we are left with a bedrock or foundation of logical premises from which we can then rebuild. Early in life this takes up a LOT of our energy.
What I am saying is; while your appreciation and interest in him might seem obvious to YOU, to him it is NOT. I am sure he has thought about it, perhaps a LOT. He has thought about your actions, he remembers your laugh and your smile, he has watched you interact with other people and maybe tried to figure out if any of your actions are unique to him.
He avoids you most likely because he doesn't want to ASSUME that you want to be around him. He doesn't want to ASSUME you like him, or enjoy his company. I did this many times. In love and emotions, we don't SEE quite so well (at first.) What someone else can point out as "obvious affection," we can quite easily write off. In a way, when he avoids you, he is gauging your reactions, seeing if you will make extra effort to engage him. It really isn't fair and it seems kinda manipulative but it's only because we have so much trouble convincing ourselves that someone is genuinely interested in us; and the thought of assuming you are and then guessing wrong is pretty terrifying, for most of us.
Like I said, he has been watching you. If he notices that you are also warm with other people, and generally talkative, generally smiley, generally whatever-you-are-with-him, he is using those observations to say "Well maybe she doesn't really like me, she's just friendly."
For you, it seems like it would send a really bad signal to be MORE obvious, and why should you? To be honest, being more "aggressive" about it is also likely to just scare us away. More of something we don't understand is tough to really digest.
I know I'm making it sound like "Well there's nothing you can do, we have broken love muscles and that's just the way it is." That's absolutely not true. You will just need to express it in a way that we can understand and be comfortable with.
We need clear communication, and we need to able to be free of expectations as to what we make of it. Essentially this means that you WILL need to tell him how you feel, and it would be best to do it in a manner which gives him the space and "air" to assess his own thoughts and feelings on the matter, without being overwhelmed by yours.
Essentially, you need so say. "Hey, I just wanted to let you know that I really enjoy your company, I am really interested in who you are as a person, I think you're super cute and I'd love to spend more time with you. I was wondering if you maybe felt the same?" Use examples if you want, be more in depth, explain things you really enjoy, all of that is fine. A lot of INTPs NEED that. We need to hear that we are valued for our eccentricities and racy minds and mildly awkward, guarded hearts. Accepting those things is the first step towards growing into them, or past them.
Express yourself, make it your own, from your heart. What you want to avoid, though? Expectations.
A) "I don't understand how you couldn't tell." / "I'm hurt and confused by your signals" / "I just wish you would open up."
B) "I'm really scared of how you might react." / "I'm going to freak out if you're not interested."
C) "Are you gay or something? Because that's okay."
Also, avoid any "games." It won't work. Hitting on some other guy, playing hard to get, being "mean" as a test; we have next to no patience for those things. Be genuine, forthright, and yourself. We might not be very well versed in matters of the heart, but for some reason we can smell fabrications from a mile away.
Case in point; I despised cheerleaders. The outfits, the chipper nods, the fake smiles, the makeup... all of it. I always wanted to yell "YOU'RE A HUMAN BEING! WTF ARE YOU DOING?!"
Make it clear, but give him an easy out. You don't want him to feel pressured by your emotions. Tell him how you feel and let him know that if he doesn't feel the same that's totally okay. You're going to have to make it easy for him to express what he really feels, and in order for him to do that, he kinda does have to feel like it doesn't affect you, or that it won't hurt you. He can't feel pressured.
Thirdly, if you want to strike up some cool conversations with him, instead of asking him how he "feels" about things (we generally get sorta "stumped" at those sorts of questions early on, internally we tend to be kinda like "it is what it is.") ask him what he THINKS about things. When we are young we tend to express our feelings in thought. We don't see them as feelings, at times. If you ask him what he THOUGHT about his sister moving, he might give you a better answer or more conversation. Even better, ask him what he thinks about something he's a bit more passionate about. Delve into deep stuff like the meaning of life, the future of humanity, why people do what they do, etc etc and you might he has more thoughts AND feelings than you imagine. If you can get him rolling about something he has a lot of thought tied up in, you'll definitely also get glimpses of his emotional self shining through, so long as he feels you are comfortable receiving it.
I know a lot of INTPs that all of a sudden feel much more affectionate towards someone just because they are having an in-depth abstract conversation. It's something very personal for us. More "day to day" stuff like who is moving, what celebrity did what, who doesn't like who, the weather, etc... You'll find us pretty dry. It's our thoughts on the big, abstract questions in life that pretty much define us. When we are expressing those things, it's like we are expressing our deepest selves. When you feel like you can connect with someone whilst expressing your deepest self, you feel very close, loving, and affectionate with that person. When someone values us for those things... well... we tend to fall in love. hahaha
And through all of this; don't lose track of yourself. Don't bend over backwards for this fellow. Don't make yourself into something you're not just to please him. Strive to understand him, but not to change yourself for his comfort. You're already perfect.
Hope all this helps, it's pretty much a giant rant.
Or am i?