[There is just something about Sebastian that screams "trustworthy" and "you can confide in me", clearly? Normally Sebastian takes pride in his ability to charm people into opening up; doesn't think it strange when it happens (goodness knows it happens often). He doesn't trust another demon to actually be so taken in by him, but... he'll respond as he usually does when wanting to inspire confidence: by listening quietly and attentively to what Rin is saying, expression thoughtful.]
I believe those are common sentiments in a situation such as yours. Growing up is difficult, after all, as is watching someone else grow up. [Or so he supposes.] No doubt even more so if it is both at once.