I'm looking up the verb "agi" and saw the following sentence in PIV: kiu agas afable, vivas agrable.

The "agrable" here confuses me since I thought "agrabla" means "pleasant". So this sentence means "He who acts friendly, lives pleasantly (as in people around him are pleased)"? I thought it made more sense to say "He who acts friendly, lives a happy life"?

Here the "base" words are the adjectives afabla and agrabla, which I have found to be such that their translation (at least to my native language, which is not English) heavily depends on the context.

For afabla PIV gives three definitions:

• Kondutanta plaĉivole kun agrablaj vortoj k komplezo
  
• Plaĉa pro la montrata bonvolemo aŭ komplezo
  
• (komputiko) Facile uzebla por nesperta uzanto

The last one is easy, "user-friendly", "easy-to-use". I think the two first ones in most cases can be translated to "friendly" and "nice(ly)" when translating to English (e.g. esti afabla al iu, to be friendly to someone; afabla informo, a nice piece of information). Note, that English and Esperanto use adjectives and adverbs differently.

For agrabla PIV gives one definition:

• Plaĉa k plezuriga

For instance esti agrabla al iu, to be pleasing to someone; agrabla vizaĝo, a pleasing face.

So the proverb Kiu agas afable, vivas agrable means, that who acts friendly/kindly (towards other people), lives a pleasant life (the life is pleasing to him/her), i.e. the good bounces back to oneself.

Note, there are three words feliĉa, ĝoja and gaja which all easily get translated to "happy" in English, while they mean different things.

... like in this example:

Por esti feliĉa, restu gaja eĉ kiam vi estas malĝoja.