Life has its difficulties as we all know and the people around you are fighting their own battles. Your friends are probably going through things they may not feel comfortable talking about. Regardless if they ask you or not, you should always be there for your friends when they are going through a rough time.
People sometimes don't like to ask for help and sometimes they want to be alone. Those are both absolutely okay (although if you're really struggling please ask for help).
Even if your friend wants to be alone, let them know that the moment they no longer want to be alone, you are there for them. Don't disappear from their life if they want a little while to themselves. Everybody deals with things differently so no one will want the same things. Disappearing on someone having a rough time might even make them feel worse than they already do. Nobody's life is perfect all the time and sometimes people want someone they can count on.
Personally, I try to be there for all my friends which can be overwhelming but it's something I like doing. You don't have to be there for everyone though because it may stress you out trying to help the friend. It's okay to just tell them you'll be there for them and that you'll comfort them when they are feeling low.
I've been in the position of not having anyone comfort me when I was having a low moment and it was awful. It made me a stronger person knowing that people may not always be there and I have to take care of myself. Luckily, I have friends now that are actually there for me.
Whether you want it or not, everybody should have someone willing to comfort them.