Lan Wangji saying such a thing is born from him missing quite a lot of social cues in his youth that lead to misunderstandings. Now he tries to be as clear as possible so he and the people he's speaking to, know where they stand. And he likes Madoka, so would prefer to make his stance obvious.
He smiles as she agrees with the statement about what friends do, making it sound so much simpler than it is with other adults.

"Well, you know that I will help you in whatever way I can, should you need it." He puts one hand a little tentatively on her shoulder in an almost paternal gesture.
"Do not hesitate to ask."