I have been struggling with the same problem you have. I understand other people 0%. Creating characters is the first step to any story and it's so discouraging to be stuck there for months or even years. Here's some things I've done to help myself, hopefully they'll provide the same for you :>
I like to follow basic story principles because they work, but because I have a hard time understanding people, I analyze human behavior, but I draw feelings and qualities from myself and things which are familiar and/or important to me. Here's what I mean.
So the first thing I do is look at something like this:
I like this one in particular because it groups needs into specific categories. The names of these categories can provide themes or morals, which is usually my focus. I then choose any number of needs from any category, but I choose ones I am drawn towards and want to explore. Because these are all relate-able human needs, it doesn't matter why these things are imbalanced (such as a financial problem, or an alien invasion), or how my character tries to fulfill these needs (such as makin mad crack cash, or kickin alien butts), all that matters is that a need is not being met, and my character is directly or indirectly struggling to fulfill it by making some kind of effort.
What might happen if my character was without comfort, or food? What sort of situation is this the result of? How does it affect their well-being? How might those needs begin to be met? Is an outside force aiding them, or providing the drive to help themselves?
Maybe I am a 6 year old child living in a rural house with my mother, and she is a hoarder, and my house is filled with junk, and I can never bring my friends over, and I have to climb over piles every day, and every night I have nightmares that the junk will eat me or I will get trapped and die, but my mom doesn't really seem to care how it affects me at all and won't listen. I have never been in that situation, but by correlating it with the list of human needs, I have the functional core of a character. Little bits like flaws and hobbies are just the sauce.
All I have to do is show how these unmet needs affect my character in the beginning, show her making an effort and struggling to better her situation in some way, and show how she changes and succeeds, or show how she doesn't change and fails, or even show how she changes but maybe it still doesn't work out in the end. It seems stupidly simple and probably like it won't work, but it gets ya every time. That's a story!
Second, I play with the nature of these ideas and how I want to plug them into a functioning story structure, using something such as this or this. The gist of this is that they are one way in the beginning, and learn something by the end. Eventually, I have enough to work with to continue on with the details of my story, although it can still take a very long time to get there. If I have a plot idea, this is usually where I'll bring it in and try to mold around it. A lot of editing and letting go of unneeded ideas ensues, but at least I have a clear idea of what I want to happen, all I have to focus on is how.
I think where I often get stuck is in "wants". When ask myself what I want, I think things like "I want to be stupid rich." But why? "Because that'd be bitchin and I could by a new laptop and eat steak all the time." --But, see, that doesn't really make for a good story. And though I couldn't even fathom the wants of other people, what I find is that when one of these human needs is off-balance, we don't realize it. It's often subconscious, yet the little desires we have are motivated by the instinct to ease that imbalance, whether we realize it or not. It also helps to ask yourself what the character is missing, rather than what they want. I could be something surprising, like compassion or closure.
Where your personal input comes in is in the "showing" part. Perhaps you have experience with a certain situation, or a feeling, or have a theme or message which is very important to you. There are no wrong answers with this, and the more personal something is (by experience, interest, or witness), the more of an impact it tends to make. Keep in mind this method is for character-driven stories only, and is much too in-depth for a plot-driven story (unless it's super long).
This is just my personal strategy and there are many many other types of character stories (for instance, they may think they are perfect and don't need to change, but are forced to by an external force). This is mainly for the protagonist and antagonist, if there are any. Side characters have a much simpler version, and their purpose is to compliment or contrast the desires of the main character. On a note, antagonists are simply characters with contrasting opinions to the protagonist, no matter if they are truly bad or good answers. (realism vs optimism, mercy vs justice, etc, or simply desires which directly oppose one another). Anyway, hope something helped and I didn't assault your eyes with my text wall :>