All the companions join forces with you because they realise it increases your chance of survival, but while of the bunch from the beach Astarion and Shart try to use you, Gale and Lae'zel treat the group as their comrades. If I remember correctly "you made an ally from Crèche K'liir" is her line when joining the team and she is true to it from the start. She still treats everyone else badly but that changes over time too.
