When luck starts rolling a certain way, it keeps building in the same direction.
Note: This is a Missing Supertrope. Luck is a confusing thing, in Real Life and in fiction. For one thing, people have a hard time defining just what luck is. If a person falls out of a three-story window and breaks their arm, are they unlucky for falling in the first place or lucky that only their arm was broken? Yes. It's difficult to judge a person's "Luck Stat" from a single instance alone, so we typically assign the quality in hindsight, to a series of events that has befallen them. If the person found a movie ticket which led them to a theater which sat them next to a supermodel who let's them move in just before their apartment building catches fire, we'd consider them very lucky. On the other hand, if they suffer through a Trauma Conga Line or Humiliation Conga, we will consider them very unlucky. This is a Super Trope for such circumstances as listed above, and it is Truth in Television to a degree. When good things happen to someone, it tends to lead to more good. Finding their One True Love can make them happier and healthier and more friendly to be around. Getting a new job or a promotion often leads to a boost in confidence, money, and status. When in tandem, these things can snowball into bigger and better things. But beware: this can work in reverse too. A bad breakup can make someone bitter, negatively impact their health, and make them a pain to be around. Being fired or demoted can result in loss of confidence, money, and reputation. As a result, things can spiral further and further out of control. The best way to explain it is that "luck" is a road, and the fork one takes can lead them closer to or further from their desired destination. No Examples, Please. Due to the Anthropic Principle, every single character in fiction can be argued to qualify in one way or another.