In another context could also be that someone is kept for himself, afraid of the world (in the sense that he/she doens´t open up to you - or to the world, to anyone.) so you are has just said something that tickled his/her heart and makes them trust you.
Now, with that sentence, the person is stating that he/she is willing to open up and face the world.
But in a given context. There is at least one different, and very common, that that Ricardo has pointed out. In a way, trusting the right people and sharing something with others, sometimes takes a weight off one´s shoulders and with that a heart will be happy/animated/peaceful (the other sense of tickling, that comes from the tickling someone´s soft spot), literally speaking.