Why do i not care when my boyfriend tickled me but it bugs me when others try to?

so in 5th period today me and my boyfriend we done with our work and their was noting else to do so we were siting on the floor talking. the teacher left the room for like 10 minuets and everybody Elas was talking to. so any way we were siting in the corner talking and he said are you ticklish and i said no (total lie im really ticklish) .he said i bet you are and started tickling me under my chin and i started laughing and he was like is that your tickle spot and i said yes but my stupid friend went and reveled that i my tickle spot was this certain spot on my side. my boyfriends like im going to find you tickle spot and so he started tickling my sides but i was wearing a shirt with a cami under it so it wasn't to bad but again my stupid friend heps by saying ha i know why its not working she wearing a shirt and a cami. so he goes o then can i tickle you under your shirt if you have a cami on. i replied no but after he begged and begged i Finlay let him and to my surprise i actually liked him tickling me. he was more careful about being to ruff with me then my other friends are. so my question are in this order 1why is it ok when he tickles me even though i usually don't like to be tickled and to what do i say to my other friend when they say how come its ok when he tickles you because that already happened twice