when you genuinely cannot fault someone- i can sit around not being able to stop thinking about someone i really like, and yet i still know she has some flaws. when i'm "in love", the person has no flaws, because all her flaws are things i actually love about her and can't see as flaws. i think i've felt the emotion of being "in love" once, and while i could come up with objective faults that she had, i can't bring myself to process them emotionally as flaws, they don't feel like flaws, they feel like part of my memory of why i was in love with her. however, like i've said i've been head over heels for other girls, not being able to get them off my mind, and yet will still perceive their flaws through that hazy mist.
i also think when you're properly in love, that person makes you a better person- they seem to take away the envy, the angst, the impatience, all those imperfections inside of you because you feel fulfilled in a way that nothing else in life can give you- except maybe in very young childhood with the snugly love of a parent. of course, this has to be mutual, else the house of cards will quickly collapse on itself, and you won't gain those positive things from that person any longer.
to be clear though, i don't think being "in love" and "loving someone" are the same. they have the same word, and are related, but for the things people are referring to when they use the terms, they are different.