Why it feels good for you
There's lots of body contact and your hands are free to play with yourself. It's also easy for him to reach around and use his fingers to stimulate your clitoris. It's fairly effortless when you're feeling tired. Most men can thrust for a long time in this position without climaxing – and it's more comfortable than the man-on-top position if he's heavy.
Why it feels good for him
It's great for premature ejaculators because it lacks the urgency of the rear position, but it's comfortable enough for him to take his time reaching orgasm. There's maximum body contact and you can reach behind and tickle his buttocks or testicles.
How to vary it
Tightening your thighs can increase friction. Grind your bottom against his penis, bend from the waist and move your torso downward. Reach down to fondle his penis while it's moving in and out of you. Rear entry in a side-to-side position is ideal for a pregnant woman, as she can support her abdomen and he can play with her full breasts.
Penetration isn't very deep in this position, so it's not for you if your man has a smallish penis.
Try it at night when you can both fall asleep in the position you made love in.