Scratch your own itch
Historically the most common reason to contribute to Open Source was "scratching your own itch". That is a person used a piece of open source software that had an annoying bug or the person wanted to have a feature that did not exist. With proprietary software one cannot not do anything. Even reporting the bug or submitting the feature request is almost impossible in most of the cases. Getting the company that developed the software to acknowledge and fix the problem hardly ever happens.
With open source it is usually very easy to report the problem and if the person has the technical knowledge then, at least in theory the bug can be fixed or the feature can be added.
In reality it can be a lot more complex than "just fix it" depending on the complexity of the project and the culture around it, but regarding the primary motivation, this is probably the strongest. So if you would like to contribute to an open source project, one of the best direction might be to find a project you use that has some issues and fix that.
I am going to go over a number of projects to see how easy or difficult it might be to contribute to that project.