Eden Hazard's youth has passed and he was the brightest when he was at Chelsea but his decision to move to Real Madrid made a complete change in addition to his long injury, maybe he was frustrated all this time? This is just my thought. 
Many reports say that Hazard prefers to retire, due to the injuries he has experienced. I think it's natural that this is the reason, he can enjoy more time with his family.
Yeah injury, it could be as he is the only one who knows about how delicate his situation is. He is more injury prone now and maybe he thinks he doesn't wants to take the risk again so he might not want to have a stint like at Madrid all back again with injuries at his new club next. It's over now what it is and I guess he made the right decision. We as football fans always seek things differently but it is always hard for the player given their condition.