Why do some guys give inconsistent attention?

I don't understand this. Why even bother putting in any effort only to later put in no effort at all? Why give mixed signals? It is such a waste of time & I find it infuriating! It gets my hopes up only for me to feel let down and disappointed. It makes me wish that the guy didn't even bother in the first place if he is going to end up acting like that. I’d rather just be left alone!

I am scared that this will only keep happening to me. I try to be a genuine, friendly person. But then guys will act like they're into me for a little while only to ignore me later. How can I prevent myself from continuing to be treated like this? I keep feeling hurt & rejected :( How can I protect myself?

Edit: also this has happened with diff types of guys (diff races, diff personality types like shy or outgoing, etc). Idk why they always treat me like this but it hurts!