Let's see, what do we know about Pyramus?
- He's young.
- He's in love.
What else? Oh! Here we go:
Wait, we said that already, didn't we? Sorry, but that's about all the myth gives us about this guy's personality. See, a lot of these ancient myths weren't particularly concerned with presenting complex characters. Instead, they're full of broad archetypes meant to be more representative than realistic.
One of the earliest examples of the young lover archetypes, Pyramus is a model for the all the passionate young lovers to follow. There's no doubt that he's an extreme example of this particular type of character. He's so passionately in love that he rashly falls on his sword when he mistakenly thinks that his love, Thisbe, has been eaten by a lioness. Maybe if he weren't so young and naïve and/or not quite so blinded by love, he might've taken a second to investigate the matter in a little bit more detail before he made so rash a decision.
But you know what they say: hindsight is 20/20.