This questions is not a common one but sometimes gets asked by customers who have a lawn tractor. 

'Why am I breaking so many belts on my lawn tractor?'

A few possible causes will be discussed below.

1) Long grass, if your grass for example is 2 feet in height (60 cm) when attempting to cut the grass it will put great strain on the belts. So something to try would be to put the tractor on the highest setting and cut the grass. Then proceed to work the height setting down in stages, this therefore reduces the strain on the belts. It is more time consuming but can increase the lifespan of the belts. 

2)Reduce the drive speed you are cutting the grass at. Instead of driving around in 6th gear if your tractor is manual reduce the drive speed to for example 3rd gear. This allows a more effective cut and once again reduces the strain on the belts.