Hardness of the cock is not the only indication the guy is enjoying the experience. I tend not be hard when getting fucked, sometimes I am, but it is not dependent on how much I am enjoying the experience. Not really sure why the difference.
Same goes for other activities in the sexual realm. I am into BDSM and can be enjoying a good flogging, bondage session, needle play session, or something else. Cock may be hard/may not be hard but I can be enjoying the activity tremendously.
So it is nice when it happens, but Tops don't let that make you think you bottom/sub is not enjoying the experience, use other feedback as well to tell you about their experience.