Should I Rejoice and Mourn?

I know I should rejoice with those who rejoice and to mourn with those who mourn. With sincere people that is a really sympathetic thing to do. I do it all the time, when something good happens to somebody I smile and congratulate them, and when they are facing something difficult, I offer my condolences. That little social transaction works very well. It does me well to follow this advice,

“Live in harmony with one another. 
Do not be proud but be willing to associate with people of low position.
Do not be conceited.”
Romans 12:16

On the other hand, there are those people who are disharmonious, proud, snobbish, conceited or something. I don’t know what it is with them, but it is impossible to rejoice with them or to be sad with them. If I hear them laughing and enjoying a moment, I might approach them and join in. The thing is, they squelch their happiness, they go mute, and practically ignore me. I don’t even think about mourning with them anymore because if I did, I would always be mourning. I think they put on a sad face, complain, and share sad news just to get a gag out of seeing my smile fade from my face. I know they are purposely “disfiguring” their faces just to look sad, just like this verse says,

“Moreover, when ye fast, be not, as the hypocrites, of a sad countenance:
for they disfigure their faces, that they may appear unto men to fast.
Verily I say unto you, they have their reward.”
Matthew 6:16

There is a day coming when Jesus will wipe away all of our tears and sorrows and fill our heart with happiness, He even said,

“…you have sorrow now, but I will see you again, and your hearts will rejoice,
and no one will take your joy from you.”
John 16:22