The purpose of Discourse in general (and of this site in particular) is to enable civilised discussions. "Yeah, I agree, but I've got nothing to add" is useful information to have for a given proposal, but let's be honest it doesn't make for a great contribution as a message. That's why you have like.
At the other end, "No, I really don't like this idea" is also important to know, but it is useless unless it comes with motivation, without something constructive. So if you don't like something, you have to speak up and explain why.
This makes for sane defaults.