The mood swings of an abusive individual can be staggering. For instance, they can go from pleasant and romantic to total rage in a matter of seconds. This extremely unpredictable behavior is almost only aimed at a submissive partner who they know won't challenge them.
An abuser wants to keep their partner feeling "off" - unsure about what's going on - and on edge - they'll do just about anything to keep the peace, and the abuser knows that.
If you're with someone who is unpredictable - who can bounce from one extreme of emotion to another - that's a huge red flag. As long as he/she can get away with it your life will be in a state of constant chaos. Someone who is emotionally stable and who doesn't feel the need to control you will be less likely to keep things stirred up and out of balance.