If my readers demand a sequel, usually that means there's something left hanging in the original. With my first M/M book, a lot of people said "I loved it, BUT...it seems like there's more to their story!!" I wrote a sequel, and it has been very popular. Another book, same deal...readers kept asking "What happens to character X?" So I wrote the sequel, and it'll be out in a few months. That's not to say the original books don't come to a conclusion. The conflict is resolved, but they don't necessarily resolve *all* of their issues. i.e., in one book, the POV character is still estranged from his brother. So in the sequel, their relationship was addressed. In the other book, there's a three-way sexual relationship, but only two of the guys are romantically involved (they have a casual relationship with the third)...a lot of readers wanted to see #3 have a chance to fall in love with someone. They ask, they receive.
In another instance, it wasn't so much that I left the characters hanging. I came up with another story, and the more I thought about it, the more I realized it would fit perfectly into the world of that particular group of characters. That sequel has been as popular, if not more so, than the first book. (And there may be a third in the series...I haven't settled on it yet)
So, in short, I've had a lot of success with sequels, but I don't necessarily write them for every book. Just depends on the story. Also depends on reader demand...if a lot of people are asking for a sequel for the same reason ("we want to see #3 fall in love!"), then I will. If it's just "I love these guys and want to see them again", then it depends on if there's any story left to tell.