That's because ranbir didn't know or understand that love isn't based on compatibility. Love is spontaneous and depends on who your heart makes an emotional connection to. There was zero emotional connection between rhea and ranbir. Their entire non-existent relationship was crushing on each other just because they thought they were similar.
And they are both good looking and popular in college and their families are close and have known each other since childhood. They got it all logical with zero emotions