The lone wanderer, I felt legitly sorry for him. I wanted to give him caps really bad; he didn't have the diolauge option though

Talk to Tommy Torini at The Tops-he will hire the Lonesome Drifter! No need to feel sad anymore!

Without spoiling anything, Rex's quest-choice-specific ending for me made me feel kinda bad. I'm a dog-sensitive person I guess.

I also felt kinda bad for the Enclave Remnants. They used to have this great vision and mission they believed in only to see it all collapse around them and be forced into living in squalor and poverty amidst the shattered remnants of their former lives.

Oh, and I feel bad for Veronica because

Rex the cyber dog?

Also how do I get him as a pet?

Ask the King about him, it'll start a quest line. The King's in Freeside.