This is a seemingly simple question, but actually is complicated. Chances are that there are many reasons that small situations become big, and also reasons that situations can seem small to you or others, but you may have intense feelings about them. If you notice this has become a pattern for you, the best advice is to seek therapy so you can look at what things set you off and why, and learn some techniques for helping yourself in those situations. It is a good thing to have control over your responses but not so great to suppress things that need to be expressed. Learning how and when to express them is important in relationships and in generally feeling better.