I lied to someone that I liked them just so they wouldn't leave me.
What made you think they were going to leave you in the first place?
I don't know, probably they think I'm some sort of asshole
It's wrong to manipulate people into staying with you by lying to them.  If you don't actually like them, then you shouldn't mind if they don't stick around.
Nah, it just makes no sense not to say I love you even if I don't. It's just a manipulative phrase that's impossible to prove, so I might as well use it so I'm not alone.
They will notice and feel it if you don't actually love them. You could seriously hurt them and damage their ability to form relationships and trust people. Is it really worth hurting them when you could be honest, find someone you really love, and let them do the same?