Patrick and Liam are biological brothers. Liam is Patrick's younger brother and Patrick loves him with all his heart. He will do anything for him. In his entire life, Patrick and Liam have never gotten into a fight with each other; their sibling-relationship is rather unique since most siblings tend to do have fights. Patrick always hangs out with Liam whenever possible, perform fun activities, run around, sing and dance (together with Finn and Brody, play Quidditch together and numerous other stuff.
Having grown up alongside Patrick, Finn and Brody, Liam always got cuddled and hugged by them. This has caused him to become somewhat addicted to hugging and cuddling: Liam always makes sure to touch them whenever he's near them. When Liam is in a bad mood or just wants to be held, he walks up to Patrick, Finn and Brody, looks up to them with his puppy eyes and opens his arm, waiting to be picked up and cuddled by them. The three older siblings have not been able to resist the "Liam Cuddle Call" as they call it.
Whenever Liam's hurt, Patrick always gets overprotective; since he is a caring person, the health and wellbeing of his family is top priority. It has probably also to do with the fact that he'll become the Head of House Dunbar one day and one of its duties are to take care of your family members.
Liam is not used to sleeping on his own as he has always slept with either Patrick, Finn or Brody (or all of them at once): they make him feel safe and protected. It's not rare for the three of them to wake up and suddenly find Liam having snuck into their room who is now clinging to them in their bed, fast asleep. Scott, Isaac and Stiles have grown used to this and even give Liam hugs as well when he enters or leave their dorm.