Happiness depends on mind, thinking, nature,situation.Some people like to spend time alone.In this time they like to think about themselves.They do their favorite jobs like singing, listening, dancing, painting etc many others thing.
In other hand some kind of people likes to spend time alone without any reason. May be he or she upset with anything. Also hurt by something. They don't like populated. They keeps themselves from everything. Its called one kind of depression.
It affects on mind, thinking, behavior.They can't share their problem with anyone. Gradually become lifeless.
But we should not ignore them. Rather we should help them to come out from their problems. We support them like a friend.
End of the day every people have right to lead a happy life. So be nice and keep others good.Stay healthy and enjoy a happy life.