If you look back there have been discussions on this point. Player countries always seem to have trouble getting alliances and, if you get them, they are rarely honoured - certainly not if the player country is a great power, and certainly not if you are the attacker, unless the ally is sphered. May be different for secondary powers. In any case, unless you have territory which another country thinks should belong to it, or you're in the way of certain colonial ambitions, then you may not need an alliance for defensive purposes (don't recall anyone declaring war on Sweden when I've been playing). If you declare war on someone else then you're on your own, anyway..
All that said playing as Germany, Russia suddenly offered me an alliance latish in the game, when my infamy got low enough.. that is, before I took another large slice out of China.. hey, I may even honour it, if its convenient..