Why do some people feel the need in a relationship to put their partner down?

I've never wondered why they do it, because in my dating days, I wouldn't stick around long enough to care.  And my husband sure doesn't do this.  I'm not being snarky here, because I've seen people get too caught up in the "why" of jerk behavior, whether it's cheating, putting their partner down, lying etc.  When people focus on this too much, they lose their focus on the fact they're accepting horrible treatment.    

My only other comment is this can be a sign of a potential abuser.  They're threatened by anything positive in your life and want to bring you down, where it's easier to control you.  If this is you, leave now while you still can.

Some guys do it unintentionally. It is called teasing, telling funny stories and jokes. Humor.  If you get a mixture of a humorous guy and a very serious woman, that problem does occur.  Especially in those societies where the rights of women have increased and they no longer accept the machismo supremo.  

Usually, the person doing the putting down is jealous of a trait/personality of their "better" half. We all at some time have baggage and invariably many "drag" it aimlessly from relationship to relationship in the hope it might justify some of their crazy toxic behaviour! My advice to you Kim is don't take any unnecessary crap from anyone as it doesn't stop there and is usually the undoing of many a loving relationship as the receiver has been deemed to have characteristics they should have had had they been born to the right parents. This flaw that they continually blame others for their failures in every they attempt to achieve will never go away no matter how patient you or anyone else is with them because you've had a better start in life even if you necessarily haven't! Why? Because you're an optimist with all your outlook within life as opposed to their pessimistic doom and gloom outlook about everything, especially you!