Even considering that Portuguese and Spanish are so similar. Why is it relatively fast and “easy” for the speakers of those languages to learn the other?

Answer 1

I wouldn´t say it is easier to learn Spanish. However, due to the proximity of both languages, in terms of grammar and lexical, it makes easier for us to read a text in Spanish or understand someone in a simpler conversation, even when we don´t have a broader understanding of the language.