Why do dogs howl?

I always wonder why dogs howl at night. Is it because there is something 'bothering' them or what they see through their eyes are 'disturbing'? I'd rather not think further so I Google'd the question and I got this answer from Yahoo.

"Well, dogs howl for many different reasons. One is to warn the pack members of danger, another is just cas and another is a greeting. Though, one can never guess what goes throuhg our babies heads......." - paintedpony72601

Thanks for that!