Except for the fact I dislike seeing anyone unhappy, it’s none of my business. Call it intellectual curiosity.
Do I understand you that the majority of Christians that you meet are unhappy, and just can't wait to leave this mortal earth? If so, that IS sad. Although I have seen that a bit in the sick and elderly, I'm not so sure they are that sad. Questioning perhaps. “Why doesn't God just take me now?”
I've always observed the Joy in Christians, even when things may not be going all that great. I must admit it always makes me uneasy though when people say they can't wait for the Second Coming. Of course I SHOULD be over-joyed at the prospect of when Jesus comes back, and I guess if I think enough on it I am.
But my first reaction is “But I like life and the world pretty much.” And also a hint of “I'm not so sure when Jesus comes back it will be all ‘happy-happy’ either”.
One of the most faithful Christians that I know is just FILLED with joyfullness in spite of some tough times. And she gets even happier when she talks about Jesus coming back; “And won't that just be so wonderful!!!??”
“Um, gulp, I guess.”
(I take comfort in the verse “And Blessed are those that are poor in spirit, for they will inherit heaven.”)