What causes cracks in stucco at 45 degree angle from top of NW corner garage door up along light fixture to wall vent?
It is a common occurrence.

What you have appears to be 2 large wall sections of stucco, connected by a small area of stucco where the crack is. For many years I thought that cement based products were very stable, but I found that it moves quite a bit itself. Usually there will be control joints added so when it does crack, with large areas it will, will be directed to the control joints. Since there were no control joints near the narrow area, it took the least path of resistance, as mentioned earlier in a post that was deleted.

Settling of the framing will do this too, but the symptoms suggest expansion and contraction, the crack is uniform in width. Typically settling would show the crack tighter at one end than the other. 

Never the less, the crack should be caulked with a good caulk, I suggest a one part polyurethane. It will need to be forced in the joint, it will be messy. To help force it in, cut the tip of the tube so the hole exposed by cutting is slightly smaller than the crack. You could mask the adjacent areas with masking or duct tape, it will keep the mess to a minimum. After applying the caulk, wipe off the excess with paint thinner or mineral spirits to minimize the line and touch up with paint unless you find a caulk color that matches the color close enough. Another suggestion perhaps, if the stucco is a sanded finish, you could have some sand handy to throw on the wet caulk to give it the texture of the rest of the wall. I have done this before with tile and concrete joints and it helps disguise the caulk.