Interest and commitment aren’t the same things. You can have an interest in something and not be committed but you can’t have commitment without interest.
Commitment is stickability. It’s what keeps you stuck in a good way—a sometimes, bad—to causes and people.
You are committed when you make up your mind about a course of action you want to take and stick with it, irrespective of how you feel later on.
The trick to commitment, however, is that you must be absolutely sure about the choice you are making, otherwise you cannot truly commit.
Interest is easy. Everyone can have interest. You can be interested in writing a book or making friends with a certain person. However, it takes depth and conviction to commit; to pound out all three hundred pages or be there for that person when they absolutely need you.
You may say you are committed to something or someone. However, you never really know if you are committed or just interested until you hit a rough patch.