The emotion gratitude is argued to play a pivotal role in building and maintaining social relationships. Evidence is accumulating that links gratitude to increases in relationship satisfaction. Yet, there is currently little evidence for how gratitude does this. The present paper provides experimental evidence of gratitude facilitating relationship-building behaviours. Study 1 provides evidence that gratitude promotes social affiliation, leading one to choose to spend time with a benefactor. Study 2 offers further evidence of gratitude's ability to strengthen relationships by showing that gratitude facilitates socially inclusive behaviours, preferentially towards one's benefactor, even when those actions come at a cost to oneself.