Well, anonymous, the good news is that this is a form of affection for a spiritomb. It feels a deep connection with you, likely due to how well the two of you work together in a battle, and it wishes to show that connection through physical contact.
The bad news is that for a pokémon consisting of 108 malevolent spirits, this is not necessarily a thing you should encourage. You may wish to establish boundaries as gently as possible—or at least carry a cleanse tag on your person at all times.