Maybe you have given yourself the answer.
The first 5-6 times was very good....then it decreased.
It is so with everything in life, our first experience is many times what we remember...both on good and bad.....but we experience it, and has nothing to compare with.
If we make a good experience often enough, then it become a normal thing to do, and we have lots of earlier experiences to put it up against, and it is very hard to achieve the feeling we had the first time, because we in reality want more.
You will see this many places in your life....and one that display it good is TV.
When one type of accident happen for the first time, hours and days goes on with rewind and rewind of the thing.
After the first "accident" will the same accident produce less focus, and in the end just give a short comment.
This is also shown by the terrorist attac, and is one of the serious ting about 9/11, because in order to grab worlds attention, they need to make it bigger, or equal, less will not give the effect they want...that is one of the reason that we need to always keep our eyes open..........but now we have come far from sex
