Why does the touch or massage of another person feel so much better than when you do it yourself?

I have terrible nerve damage / pain and numbness in my right arm and hand, and when I had physical therapy or when my GF massages this area, it feels excruciatingly good. But I cannot do this myself (even though physical therapists have encouraged me to do it regularly). It has to be another’s hand.

This experience is fairly common with people who undergo physical therapy or regularly have a good massage or back rub. Which is ironic, considering that we rarely completely trust another person with wounds or injuries.

Touch can also be stimulating, healing, and supportive in ways that are hard to define. But we cannot replicate the effect on ourselves. It has to be another person. Why is that?