If I'm trying to understand what Horace is trying to communicate... I think he's trying to say that one's perspective on life can greatly influence how they experience it. Those who think critically and analytically may find humour and irony in difficult situations, while those who have a strong emotional response to events may find them to be more tragic and difficult.
a) This is terrible! How am I going to pay the rent now?
b) What a relief! That place wasn't for me anyway. Now I can focus on finding another job and going for interview without having to pretend I'm not find another job! lol
Someone disagrees with you on a forum:
a) WHAT!?!? How mean! How dare they? No one ever understands me!
b) Interesting point! I'm glad people here can broaden my thoughts a bit. Go free speech! Maybe I gotta work on articulating my thoughts better! They're right... I guess I'm not that smart!
What is life?
What does life mean to you?
Do you consider life to be [primarily, or, defined by] existence, being, and body, or mind, spirit, and soul?
Do we have only one life or multiple lives?
Is life mostly being or becoming?
The quote can be interpreted in whatever way the reader prefers and there's no standard answer. Most educated people probably are capable of coming up with some bite-sized philosophy like this. But before we explain the quote, it might be a good idea to look into some more fundamental questions.