Mmm~ chemicals do different things to different people. Well, they generally do the same thing~ but perspective of those chemicals are what differs between people. Tickling releases a lot of endorphins. There's a rush depending on the context. ^_^ I could say in itself, tickling can be pretty stressful in itself! (Tickle fight! XD)
...but I think any bonding done in a fun way where you're safe, and can actually "progress" your emotions can be stress relieving. I've tickled publicly~ with complete strangers at parties out of the blue, and due to the already good moods floating around a person can go from "Oh no~ I'm being tickled to..." ~ "Well... it's okay, I like it." X3 Without shame~
I think when you have the space, respect and time to deal with any kind of emotional transition, there is release. Sexual or not~ It's like the transition from being stressed on a job, and the relief of being done. You need that good flow from one state of mind to the other for it to ever be called "relief", and it can be instant, or take a few minutes...
Either way I'll tickle them till it happens~! X3 Nyehehe! (Jk, sorta.)