According to jisho.org, だって has these meanings:
- after all; because
- too; as well; also
- they say; I hear; you mean
In my opinion, "because" and "but", and "as well" are completely different things. I understand that the actual meaning of だって is inferred by context, but having so many distinct meanings is kind of weird.
I know that だって is short for だという, which means roughly "[some pronoun] say". That is the meaning number 5 of だって.
How does "[some pronoun] say" turn into "because", "but" and "even" etc?