Why, in your opinion, did sheila agree to go out with the narrator even though her behavior indicates that she is not interested in him?

In my opinion, Sheila is using the boy just to have fun.

Sheila is displayed in the writing as a pretty self-absorbed young lady who doesn't want to tied to any responsibility.

For example, on the canoe she said "I'm playing outfield ... I don't like the responsibility of having a base."

This is a metaphor that she used to say that she does not want to be tied to other people in relationship.