They are in love with each other.
They are close in age.
The friar thinks that marrying them might eventually end the feud between their families.
Their families are fighting with one another
They are both too young
They haven't known each other long enough.
Romeo is on the rebound--2 hours before he was whining about how he loved another girl and she didn't love him back
Romeo is emotionally unstable. He has depressive tendencies.
Both of them are too impulsive.
Their parents won't approve.
Juliet's father has been negotiating her marriage already with someone else.
They could end up dead.