It's normal to feel sad from time to time, but if you're sad or numb most of the time even while participating in fun activities, you may be suffering from high functioning depression.
I think it is quite important, obviously if it's your, like your mother's or your girlfriend's or your boyfriend's birthday and you don't give them a gift, then I'm sure they are going to be quite upset.
When we cry, our top lip starts to wobble and so this gave rise to the idiom a stiff upper lip, meaning to not show your feelings when you are upset, even though it is difficult not to.