What a great question! Yep, we are usually the cause of our own resentment, at least in my experience. That doesn't mean we have necessarily "caused" an unpleasant occurrence in the first place, but how we interpret the event and our future options doesn't always include working it through, or forgiveness. We probably learn a great deal of this in our childhoods, watching adults model supposedly adult behavior and resolution.
My mother, for example, could hold forth at length about how much she resented us kids (in the days before reliable birth control) for destroying her life plans, yet she would not tolerate an angry word from any of her 4 daughters. What was left for us as little kids but simmering resentment? I was in high school before I dared express anything but loving adoration of my mother, or developed the guts to argue with her about anything. (Not that I ever "won" – her temper was formidable.)
And yet, I've recognized throughout my adult life how important it is to get over that, and not for anyone's sake but my own. It has taken a great deal of reading and some counseling to help get me past my learned pattern of silently simmering. And my loving and patient second husband, with gentle honesty, helps draw me out when he can tell I'm stuck on some negative reaction. I'm getting better at expressing what I'm feeling, but even at almost 66, I'm still working on it.
You're right, it's not healthy. In my case, I've learned that suppressed anger (or fear) can bring on a severe asthma attack. As I've gradually learned to express myself more clearly and honestly, I've been able to drop the use of a very expensive inhaler that I had to use for many years. Yay – evidence that I'm making progress!
A particular process that has helped me with this is Byron Katie's "The Work." Sincerely answering 4 simple (but not always easy) questions:
1. Is it true? (my judgement about a situation or person)
2. Can you absolutely know it's true?
3. How do you react/feel, what happens inside you, when you have that thought?
4. Who would you be without that thought? (this can be mind-blowing!)
For more on this pathway: http://www.thework.com/thework.php
ADDED: OnePerfectOne reminds me that in my 20's, I found Transactional Analysis (as in the wonderful book I'm OK, You're OK) was transformational for me, and the beginning of my real growing-up.