To make a well rounded character you must feed them lots of sweets, and they must be complex. One way to be a complex character is to have motives for your actions. Why does Billy do this? Why does Johnny do that? When given the same plot, similar characters will deal with events differently based on their own motives. Short term goals, long term goals and desires can be motivations for your characters.
When considering motives for your characters you can determine motives prior to writing your story such as in the initial stages of plot outlining and backstory writing; you can watch as your character interacts with other characters and follows the plot to determine their motives, or you can take another path entirely which I haven’t outlined here.
Also, motives can help to connect your characters to your readers. By establishing believable motives, your characters can obtain sympathy from the readers of your work, and this sympathy will keep them reading.
If you have trouble writing up believable motives for your characters, think of your own motivation. Why do you get out of bed each morning? What are you working for? Also, consider if you were in the plot of your own story, what would your own motives be? Would those motives fit the motives of one of your characters? If you were tossed back in time by a crazed scientist would you do the things you do because you want to get home, or would you do different things because you want to screw with the past (although it's possible that everything you do in the past you already did and anything you do won't actually change the future - this depends on whether or not time is linear or circular. Being a teacher in the school of "time is circular", myself, my understanding is that if you were to go back and kill yourself and you would find out that you did not kill yourself because if you kill yourself then there is no you in the future to come back to the past and kill yourself. It's like getting yourself into a pickle you can never defeat. - Good times). Ultimately, the question in this scenario is, are you motivated by good outcomes or bad outcomes. Ultimately, as a writer, are your characters motivated by their own desires or by the well being of others?
As a final example, let us consider the character Huey Lewis. At one point he was motivated by his desire to "find a new drug", another time his motivation came from "the power of love". Now ask yourself, are these motives believable? Do you have sympathy for the character of Huey Lewis? I think we can all learn from this relatively flat character (almost one minded, if you ask me) and we can make our characters stronger. Just remember the mantra: "Better than Huey Lewis". Whenever I feel down, I tell myself that I'm better than Huey Lewis and I feel great! Some days it takes a "I'm so much better than..." or "I'm amazing and that Huey Lewis is a dud," but no matter what it takes, it works. And it can work for making your lives better and the complexity of your characters better too. So next time you're working on a character and they don't have good enough motives, just ask yourself "how cany my character be better than Huey Lewis, just like I am?"*
*With no disrespect to Mr. Lewis or the legal force he has working for him. He's a great guy and I love his music.