Answer by Visakan Veerasamy:
They are loyal and kind, but they have very little self-control. They're silly and just want to have fun. They love their boss, they love playing, and they get into trouble and screw things up.
They get caught up in petty oneupmanship and childish jokes, bu they also try their best to do the work that they're tasked to do. They want to be kissed goodnight by Gru.
They're adorable because they're "human", in the sense that they're imperfect. They're given far more personality than you'd expect from "minions", which are typically bland and neutral. (This was also why there was that scene in Iron Man 3 where the henchman said "Honestly, these guys are so weird." was so funny).
We get to see them struggle with self-control, struggle with trying to be good, responsible. They mess up but they try. They enjoy the little pleasures. We relate to them, so we love them.