You are a nice person. That is why everyone perceives you as a
happy and cheerful person, and a person whom everyone can go to
when they are down. Well, perhaps you are sensitive enough to
detect when your friends are feeling down, but others are just not
as sensitive. But it could also be you are always clearest about
how you yourself feel - nobody else can be clearer. And feeling
down seems to be more frequently occuring to you than to your
friends, so it does seem that when you feel down, hardly anybody
notices and cares.
By the way, when you bring that smile to your friend's face when he
or she feels down, did he or she feel you did enough to cheer
him/her up? So likewise, when your friends did try to cheer you up,
you may feel it's not enough or their actions were not even noted
as 'cheering you up'. So it is about management of expectations.
You must understand that you know yourself more than anybody else
does - not even your best friend. I know how some people claim that
their friends and families know them more than they do, but it's
just a matter of what is more obvious to who. (For example, your
friends and families are actually your 'mirrors', seeing things
that you wouldn't be able to detect immediately for yourself.) At
least in terms of your own inner feelings, nobody knows better than
yourself. So it is not
fair to expect others to be able to read what's on
your mind, and what you need in order to really make you feel
better.
Well, I choose to take the possibility that you are a more
sensitive person while your friends wouldn't be able to tell that
you need to be cheered up, or what effective methods to use in
order to cheer you up. As such, the first breakthrough is to tell
your friends when you feel down - explicitly. And during
opportune moments, do let them know too, how they can cheer you up
when you feel down. You can bring this across in a more jovial
manner when there's the chance.
Meanwhile, love and care for yourself. You cannot expect others to
love and care for you when you cannot even do it yourself. For one,
how are you going to tell your friends how they can help make you
feel better if you cannot even identify the ways and methods that
are effective to you?