I think that this situation is very common. My WAH says something similar about not voicing how he felt because he thought my reactions were going to be so awful. That led to a great deal of resentment building up in him that he says he can't get over. Unfortunately, because he wasn't being honest with me that he wasn't happy, I thought we were doing great.
My counselor said it is a case of pursuer/distance type of relationship. So, I just ordered two books to help me figure out my role in this debacle. I am considered the pursuer (codependent) in our relationship. I think that would be what you are calling the "nice guy" in your case. I am not sure these books will help me with my M, but I don't ever want to make the same mistake in my next relationship with whomever I am with. So, I am anxious to see if anyone else weighs in on your question...
M 44 H 46 M 20yrs T 25 yrs S15 S12
ILYBINILWY 7/18/15 Move to MBR 9/8/15 Physical Separation 10/10/15 Suspect A 8/2015 Confirm A 12/27/15 D filed by H 2/2/16