PSSDorAmINormal wrote:(edited for focus)
I have often wondered why I can't seem to enjoy internal stimulation or find pleasure in it the same way I do the external way. What would cause this?
A person's skin is loaded with nerve endings which vary in density all over the body and even varies from person to person. Also, psychological aspects of stimulation (especially location on the body and context of the contact) will make a difference to how you feel about the activity.
My girlfriend loves to have her back scratched. I don't. I like a light, feather touch. By the same token one man might like a heavy touch on his perineum and another, a light touch. Some men might respond to pressure on the prostate from the inside and another might like stroking better. Also, one can "teach" one's nerve endings to like being stimulated in certain manners (that is, by repeated stimulation in a good context, a pseudo-pavlovian response of pleasure can be developed).
As for me, my perineum and scrotum have become more responsive/pleasurable over time as my currend girlfriend enjoys stroking me there (more than previous lovers have, so has generated a more intense response from me as she has done it over the past couple of years). My GF and I are experimenting with prostate massage to induce a "P-gasm" for me. I think we succeeded once and I look forward to more experimentation. But the P-gasm has proved elusive so far. I have done this with women to induce a G-spot orgasm, which for some women must be "learned" and for many is elusive as well. Technigue of the touch (stroking, pressure, speed, etc) must be practiced diligently.
So, to answer your question "what would cause this" (difference in pleasure); all the details and peripheral psychological, physical and emotional factors will make differences. The good news is that many of the factors that affect your response can be influenced or changed by practice (on the part of your lover and on the part of yourself, too).