NO SPOILERS! My guarantee to you.
Han Solo's motivation when we first meet him in Star Wars IV: A New Hope, was greed. He wanted as much money as he could get, and fast. In order to make money, he became a space pirate. The life of a pirate is synonymous with that of a hustler. And the outlaw lifestyle cannot be sustained long by an honest, goodhearted man. Han Solo adapted. He became what many people would be like if they had to hustle to survive: devious, boastful, and as a hazard of the job, proficient with a blaster.
None of these things really describe Han Solo as a character, however. They just describe what he does because of what his motivations are. I doubt anybody wants to be judged solely by what they do in real life. I recall reading a study where most people described themselves as "a good person" regardless of their walk of life, or regardless of their past deeds. Likewise a character in a piece of fiction would probably describe themselves more kindly than their actions alone might suggest.
Fictional characters should have goals, dreams and a trajectory they'd like to follow. And we want to watch them as their trajectories are disrupted. How they cope, and how they change, are definitely influenced by their motivations, but also by their circumstances. This is how they arc as characters, and this is how we get to know and care about them. (Han doesn't want to get involved in the war between the Empire and the Rebel Alliance, but over the course of A New Hope, we see him change.)
But some things that make us love them as characters have nothing to do with how they change. Some things came before we met them, and they don't have to change unless it's relevant to the story. Setting aside the wonderful things Harrison Ford added to the Han Solo character ("I love you. "I know.") there were things about the character that were written in from the start.
Han Solo is smarmy and arrogant. He's impatient and sometimes rude. He's got no qualms about taking another's life (Rest In Peace Greedo). He seems to have one friend in the whole galaxy, and we get the feeling it took a long time to build that relationship. The aforementioned exchange with Princess Leia only serves as further proof that he's got a fear of commitment. In short, he's flawed.
Part of what makes characters memorable, and what makes them work, is their flaws. And that's flaws, plural. It can't be just one. Even if their flaws stem from one event, there is rarely one symptom of a sickness. Their shortcomings and traumas have to manifest in multiple ways. As an audience, we know ourselves to be flawed in many ways, and that's how we sympathize with great characters.
Think about Martin Riggs from the Lethal Weapon series. Sure, he's brave, funny, badass, and governed by a strong sense of right and wrong. But that would make him seem like some inhuman Golden Boy if we left it at that. The flip-side is he's prone to extreme violence, overreaction, homophobia and suicidal tendencies. His motivation is his morality, his trauma is the death of his wife, and his resulting flaws have accumulated from the 30-some-odd-years his life developed before we meet him onscreen.
Action movies don't always have time to be character studies, but without at least some development, the actions of a flat character also tend to feel flat. This is one of my most frustrating issues with Star Wars VII: The Force Awakens. Aside from old characters we already know and love, the characters of this film are mostly two-dimensional. Rey, Finn, Maz Kanata, Snoke and Hux have motivations, but nothing else. Poe Dameron is funny, but then he disappears for most of the film.
Kylo Ren has character development for days, but on this he stands alone. He seems at first deadly and frighteningly calm, but then deteriorates into a tantrum-throwing spoiled brat. This shows another characteristic: subterfuge. It's believable that he would try to hide the emotional turmoil he's in when it's go-time on Jakku. Something happened before we met him that took him from the light side of the Force to the dark side, and he's still struggling with it, with devastating results to all caught in his wake. He is a character who is actually unsure of his motivations, which is a refreshing spin.
Compared with Han Solo, Kylo Ren has even more going on. So even though his character is inherently unlikable, he's still incredibly well-developed. It's as if the writers put all their effort into fleshing him out, and perhaps ran out of time to work on any other characters.
There was plenty more I found problematic with The Force Awakens, but I wanted to highlight character development as a significant culprit to show that millions of dollars cannot make the audience care about anyone. Only good writing and good execution can do that. I'm hopeful Star Wars VIII will be better.