My spider is climbing... why ?

If you have an arboreal spider, then climbing is what it does, and that's all fine.

 

If your terrestrial, or burrowing spider is doing it, then this is less normal. Questions to ask include:

Asking these sorts of questions can point you in the right direction, and help you find out why your spider is climbing. Or, it may be that is just the mood your spider is in that night that makes it want to wander the glass. Only careful watching will give you the answer...