Ernie mans the soda fountain.
"I'm going to drink this ice cream soda for you, Bert!" Well gee, Ernie, don't put yourself out!
With that said, I do think that Bert is kind of a jerk in this sketch. After all, Ernie is just trying to do something nice for him, and at the end Bert gets exactly what he wanted the whole time, so what is he still complaining about? By the way, that bit where he asks Ernie to come in close and then yells at him is a common maneuver for him, and it's always hilarious and effective at getting the point across, to the viewer if not to Ernie, but it's also kind of mean.
Nevertheless, I crack up at the way he says, in injured tones, that he just wanted a "pure, innocent" drink. And I do have sympathy for him because there are a lot of things that I like to eat plain to which other people feel the need to add dips, toppings, spreads, condiments, garnishes, etc.
This sketch gives us some insight into Ernie's character that we also saw come into play in the "friend-watching" sketch a week ago. Ernie's intentions are always good, and he is very good at following the Golden Rule and treating people as he himself would like to be treated. The problem is that he has a hard time seeing other people's point of view and treating them how THEY would like to be treated. Of course, that's where the lesson comes in; a lot of preschool-age children have trouble with that too, and so do a lot of adults, now that I think about it.
At one point in this sketch, Ernie gets ice cream on his upper lip. I love that. I think it was probably an accident, but it heightens the illusion that he was really drinking it.