Here's how I think about it. First, I thought about what makes humans attracted to each other. What makes them grow close.
I think it's several things, but one is secrets, another weaknesses. When we share a secret with someone, something personal about our lives that we don't tell others, they feel special. They feel loved, trusted. This makes a stronger bond.
We can use this to our advantage in stories. Have the MC share a very private matter with another character, something they don't tell others. As a result, the reader learns the secret and feels closer.
It could also be done in narration.
He was five when he shot the bird, never sure why, but he did it. To this day, it gives him nightmares, although he hasn't killed a living soul since. He never told anyone and probably never will.
Learning about people's weakness makes us closer to them as well. For example, if I'm on a date and reveal a weakness, I actually gain a bit of her trust. Not much, but I do, and that's the first time I met her.
This can be done in narration or showing it in scenes.
Fear of heights, such a common thing, but it shamed him. He tried hard to fight that fear by crossing bridges whenever he could; even though, his hands sweated and his vision tunneled.
You could also show the above in a scene.
1. Secrets, privy information
2. Weaknesses
Two more are bad past experiences, like a hard life, and talents.
Why do we often feel closer to an actor than a stranger, or a singer than a stranger? Talents. For whatever reasons, we are attracted to talents.
You can show your MC's talents in a scene. Maybe they sing well, or dance well, or draw, etc.
We tend to grow closer to people when we learn about their hard life growing up. Before that, we might not even feel sorry for them. We might see a killer on the news and be disgusted, but then we learn about the killer's hard childhood, and we can't but help to feel a bit sorry for him. His father raped him. His mother abandoned him. He was shipped from foster care to foster care, no one ever showing him love. He was then on the streets at the age of 14, etc.
After reading the Shoemaker, I couldn't help but feel sorry for the serial killer Joseph Kallinger, and his son. Great book, BTW.
TV shows like the X Factor use human psychology to their advantage. They show us a bit about the singer's life, their goals, how much they want that goal, and some of their struggles. This builds sympathy, and we want them to win.
Grandmother paid for all her granddaughter's singing lessons. All she wanted before she died was to see her granddaughter succeed as a singer, but now she's dead, and the granddaughter is on X Factor. She's doing this for her grandmother. She will win. Her grandmother is up there in heaven, watching. This is for you grandma. I'll do it for you!
Stephen King mainly focuses on their real-life struggles: relationships, kids, rent, losing the house, alcoholic husband, etc. This gives us a sense that these are real people with real issues. We can relate to them. Then he throws them in a bizarre world, but because we feel the characters are real, we accept the horror that befalls them. We want them to succeed.
I hope this helps.