Could be: Wrong contactor for application,poor connection causing overheating,poor coil voltage causing it not pull in tightly,too frequent compressor on off cycles,ect.
They should last a long time.

We had the same problem last year, I didn't know what it was, but it did end up being the contactor as you mentioned. It was a brand new unit, so I new something was wrong right away. Since I was new to town, it took me awhile to find Louisville air conditioning contractor to fix it, but he knew exactly what to do, and my husband was so relieved.