Stiles and Patrick officially met each other during the Sorting Ceremony in 1991 where the two boys were sorted into Hufflepuff and were assigned to the same dorm, becoming roommates with Brody Hudson, Isaac Lahey and Scott McCall. Patrick instantly liked Stiles when he met him as well although he had to get used to the hyperactiveness that seemed to come off of him. Stiles is someone that never fails to make Patrick laugh whenever he's feeling down and the two are always there for each other.
Patrick has a very special connection with Stiles: he often comforts him when Stiles is feeling sad about the death of his mother, since he can completely relate to it since he has lost his real mother as well. However, Stiles doesn't need to comfort Patrick because he's not sad about it at all: he has Carole Hudson who he sees as his true mother so in a sense, he has never really lost anyone.
In his third year (before he got into a relationship with Theo Raeken), Patrick helped Stiles out when he was depressed that he still hasn't had his first kiss yet. When Stiles was sad about it, Patrick grabbed Stiles his face when they were alone in their dorm and kissed him, surprising Stiles at first who eventually ended up participating into the kiss after he got over the shock. Without thinking too much of it, the kiss soon ended up in them having sex, making Patrick Stiles his first as well. Afterwards, the two of them act as if nothing ever happened between them although he has a feeling that Stiles might want to since he's sitting with him now more than ever, touches him more than necessary and tries to get alone with him in a room as often as possible.