Question

Why is it not always necessary to include a subject pronoun before the verb in Spanish ?

Answer 1

It is not always necessary because of verb conjugation. Verb conjugation refers to the process of changing a verb form to provide information about the action being performed. The verb can give us some idea about who is performing the action, as well as when the action is being performed, and the relation of the verb to other parts of the sentence. 
I hope this helped you i was in Spanish for a few weeks starting August 24th this year but now since I'm home schooled I'm not in it. But that is what i remember my teacher telling us about.

Answer 2

It's important to remember that in Spanish the verb forms often make subject pronouns unnecessary, and when that's the case the pronouns shouldn't be used unless there's a reason to.