Why is it so hard for a higher belt to tap to a lower belt?

I have noticed a common saying among Jiu Jitsu Practitioner and that is leave your ego at the door.  I am starting to think that this is a one sided proposition.  It seems  like they are telling the new guys to not get fired up and go all crazy when a higher belt takes you to school and works you over.  I mean most of the higher belts take it easy on me.  No one really wants to hurt me or actually hurt anyone else when they are sparring for the most part.  The only time I see this change is when you catch a higher belt in a move that they have to really fight to get out of and then they go crazy on you trying to show there superiority.  They don't really have to do this as I already know they are better then me and so they have nothing to prove to me.

So yesterday I was rolling with a Purple Belt.  He was 20 years younger then me weighed about 50 lbs more was much stronger then me (or anyone at the gym for that matter) and is even better looking then me.  As we rolled I could tell he was working hard on trying to be technical and not just use his brute strength.  He has hurt me in the past using strength so I think he try to calm it down with me and works on technique mostly when we roll.  So yesterday we were rolling and I caught him in a very tight ankle lock.  It was a straight ankle lock and I could tell I had him.  I looked at his face in pain and realized he was not going to tap.  He tried to roll through but it just made it tighter and he still was not going to tap.  He tried to go 50/50 with me but it was not happening.  So I figured if he was not going to tap I was just going to let it go.  No need to hurt him because of his own stubbornness.  I know some may have an issue with this and may have wanted  me to finish this but I thought why hurt the guy just because he is too proud to tap to an older lower belt then him.

What do you guys think?  Is this a common practice where it is hard a higher belt to tap to a lower belt or someone they think has inferior skills to themselves but if that person is a higher belt or someone with better skills you would tap during the same hold and the same pain level.