What's a "dealbreaker"? Some quirk, trait, habit, etc. in a potential mate that's so irksome that you just walk away from the relationship. For example, maybe:

She constantly talks about her ex.
He’s a major Trekkie.
She doesn’t shave her legs.
He kisses his mother on the lips.

Read these and other dealbreaker scenarios and decide where you would draw the line (although it's usually more fun to quiz your friends and see what would be a dealbreaker for them).