Guilt
I have always believed that feelings are messengers we need to pay attention to, even if they are unpleasant. Sometimes, for reasons I will look at briefly in a minute, they become overwhelming and harmful, but very often we just try to shove them away if they make us uncomfortable.
After all, being happy is what it's all about, right? So, all we need to do is think positively, be nice, make gratitude lists, and believe that life is all sweetness and giggles. Said no-one.
Guilt is one of those feelings we try to disown, sometimes with good cause. If it is undeserved guilt- survivor's guilt, for example, or habitual negative thoughts, or the result of continuous undermining by an abuser. This is definitely unhealthy, and can be a sign of depression, or be connected with PTSD. It can lead to self-doubt and low self-esteem. With something like that, professional help would be recommended.
There ARE times, however, when guilt is useful, and deserves to be acknowledged. 
Guilt can heal relationships and preserve social bonds. If you did something that hurt someone, or even something that felt that you violated your own values; or you DIDN'T do something, with a similar result, you feel guilty. (If you know the picnic scene from "Emma", you can see this vividly portrayed there!) This can lead you to apologise, to make amends, to resolve to change the way you behave in future. But in order to do that, it has to feel uncomfortable.
If you have codependent traits, guilt follows you around like a bad smell. You feel guilty if you don't put someone else first. If you are in a relationship with a manipulative partner, that button is an easy one for them to push.
Here's a different take on it. For many people, guilt is a sign they have chosen to do something for themselves. If you constantly make the needs of others your priority, you feel resentful.
"If a refusal saddles you with guilt, while consent leaves resentment in its wake, opt for the guilt.Resentment is soul suicide"
(Gabor Mate "When the Body Says No")
