Don't suppress your emotions in relationships
Why would people do this?
For one, they may want to avoid having an argument. Or, they could very well be hoping that whatever situation is triggering those emotions could be resolved before they have to come out and express their feelings.
But such a strategy is misguided.
Keeping those feelings bottled up might only lead to hurt feelings and resentment later on.
Whether you're feeling sad or angry, opening up to your partner will only make you feel better. One of the reasons we enter into a relationship in the first place is to have someone we can confide in -- someone who will hear us out and provide support when we need it most.
By keeping your feelings to yourself, you prevent your partner from connecting with you on an emotional level and, even worse, may engender suspicion in your significant other that something more serious is afoot (cheating, etc.).
It's better to let those emotions come to the fore than allow them to fester and wreak havoc on your life. Hiding behind a facade of happiness is not only disingenuous to your partner, but to yourself as well.
In the long run, emotional suppression is damaging to the long-term quality of a relationship.
When it doubt, let it out! You'll feel like a load has been lifted from your shoulders.