"what's causing it and how can i remedy that? will the fan burn it's motor out by it's constant operation?"

1. The usual cause is the heatsink compound needs to be refreshed or rather replaced. I'll tear it down, clean off the old, put on the nice ARCTIC SILVER (see google) and then button it ups.

2. Fans have a "lifespan." If it burnt out due to being on then it just needs to be replaced. Figure 1 to 2 years for your average fan under average conditions. Figure 5 to 7 years for your better fan in a clean machine.