It's difficult I know at funerals to be happy, they're not conducive to that state of mind, however at the one I attended today the minister said that it should be a celebration of the life passed away. This makes sense. Your friend is gone, but you shouldn't dwell on this, remember him/her for what they were, remember them making you laugh, cry, happy, angry. You may have lost someone physically, but their influence on you is part of who you are now. And that should be celebrated.