How does one just pretends to be happy ?

The difference between those who are happy and those who aren’t is largely determined during the difficult times in life. Happy people choose to see these periods as opportunities to learn and adapt. This is done by consciously choosing to focus on all that is good and right in life at the time instead of dwelling on all that is wrong.

That's what many people do, cause one doesn't really want their family or anybody to worry about them. But sometimes one wishes to knew another depressed, social phobic person so one could talk to them or at least know somebody who goes through the same thing as them...not that one would wish depression on anybody but one just wish to knew somebody else who had the same problem....cause anybody else just either worries about you or they don't understand or care at all.

 
Sometimes, when one is really sick of feeling like crap, one do pretend, because its said that if you pretend long enough you might start to believe it and it will be true. 

Though that doesn’t work for all.

One is also extremely worried about how their behavior affects those around them, and when one starts to notice that one is making other people sad they tend pretend to be fine. 

Being outwardly depressed also causes people to pay attention to you, sometimes you'll get people who want to cheer you up, and all that attention is scary! So one just lie and say, “yeah, they all are better now”.

It's easier to be one person in front of people and ones is regular sad self behind closed doors. But then one start to feel isolated because nobody knows the "reality", nobody knows or understands ones reality. It's just a big vicious cycle.