If I think someone's cool I'll sometimes not know what to reply that won't make me seem to stuck up, too insecire, that I want to be friends more than they do, that I am bored by them and that makes me come across as someone they wanna be friends with too.

So after type and erasing different messages over and over I'll give up for the day, get new messages and forget I never sent anything

If it's opposite sex then it could be jealous bf/gf who throws a fit when they see them messaging you, they have a big argument and promise not to talk to you anymore but they never tell you because they are ashamed of their bf/gf behavior l.