My first boyfriend seemed to never get mad about anything, then one day, he just flipped and broke some things and screamed at the top of his lungs.
Later, he apologized, but I noticed this was the same anger pattern that his dad had. This happened a few more times over the years, and it was really scary.
We ended up breaking up, which was the best move of my life. But looking back, the red flag was this lack of anger and lack of discussing things that obviously had bothered him.
It is much better to discuss things as they come up, and not let them bottle up. This can make for a bad relationship.