Why did I watch SHIELD? There are 3 huge reasons.
The first was the character development. Fitz went from meek scientist to becoming a leader, look at season 2 when he stood against the entire team to defend Daisy. May went from agent with reputation for crushing people and the cold exterior to match, to being Team Mom, and being damn proud of that role. Daisy went from hacker who didn't really fit in, to finally having the family she never had. Like Daisy said "I already have a sister to save, her name is Jemma Simmons." Hell, at the beginning Daisy could get annoying, but they turned it around with her character. Even one season characters like Jaco go it, Jaco went from following Sarge to hating Sarge's tactics and the fact Sarge left Snow behind, he became the baker that he truly was, and went out as a friend of SHIELD.
Secondly, they weren't afraid to give us the happy moments without any pain. You always fear that the happy moment will be crushed. SHIELD gave you the happy moments. We got Cal getting a happy ending, May and Coulson's kiss, Fitz and Simmons getting married, and the finale.
The biggest reason was the chemistry of the cast. Everybody worked well together, it didn't matter the pairing. It didn't matter if it was Daisy and Fitz, Mack and Simmons, May and Daisy, or anyone else, you believed they were a family.