The seller likely does not want to waste his time or yours if he knows his house is not up to their standard.
Have you checked the transaction history of the house? Someone may have already been denied an FHA loan on this particular house, and the seller may be aware of it already.
Also, it sounds like Seller's were entertaining other offers & chose another offer who had more money down or something else that made them feel more secure.
In general, people are only emotionally connected during the real estate transaction when it comes to their own "bottomline" numbers & demands.
It sounds like you & your spouse are on a great path!
One suggestions was that the seller may think that the FHA inspection may pick up on something. The roof is just about at its life span.
I agree with Fred. If after three counters they left the negotiations, it would indicate to me they received a better offer. If they were going to decide not to accept your FHA financing, they wouldn't have engaged in counter offers with you.
Best of luck with your house hunting. Don't give up!