It is difficult for people to deal with a friend with mental illness, because they probably do not understand what is wrong. and although their intentions will be good, they may end up coming off as insensitive.
Telling someone they cannot feel sad or insulted because other people go through it too
and are probably worse, is like telling someone their happiness isn’t justified because other people have it better!
Be there for your friends in their time of need, instead of not noticing their needs all together..you should help them get through it, and for that you need to accept the existence of the problem in the first place.
If people have the right to celebrate their achievements in life, people also have the right to be sad, feel dejected, and vent out every once in a while. As a friend for someone going through a rough phase, it is important to let them release their feelings, instead of making them shut them out. That maybe be a short term solution, and probably helpful if they are in a social situation like a function or school etc. but it is not an advisable prescription.
Because days later, they can get triggered by something minuscule, which would then release into a volcano of pent up repressed emotions which were forcefully shut down.
Be there for your friends, they need you more than they let you know.