Why is my propane tank leaking?

I have a 500gal in-ground propane tank that was installed 3 yrs ago. I was told today that it is leaking and he gave me no reason as to how this happened. It is leaking at a seam below the guage. I was told they would have to weld it and that it is very laborous. I am wondering what would cause this and why I would have to pay for repairs..