There a lots of differences between professionals and hobbyists at anything, but there's one thing that crops up time and again, and it appears to be particularly true of those in creative professions.
There are undoubtedly a plethora of differences between amateurs and professionals in photography or any other field for that matter. If you ask people which ones are the key disparities between the two, you'll get as many answers as there are cameras. You'd be hard pushed to find anyone who would claim it comes down to gear. You'd be equally hard pushed to find a professional who believes it comes down to talent; there are spectacularly talented hobbyists and highly underwhelming professionals. The real difference is something that every professional I've ever met in any creative career has and that hobbyists do not: a specific mindset.
The mindset has been expressed in lots of different ways, but I believe the clearest way is by two quotes. The first is Stephen King: "Amateurs sit and wait for inspiration, the rest of us just get up and go to work." The second is Henri Matisse: "Don’t wait for inspiration. It comes while one is working." The bulk of the information you need is from King's words of wisdom, and his equally wise actions. It is said that King writes 10 pages per day, every day. When he's working on a project, he'll write around 180,000 words in 3 months. He doesn't sit around and wait until he's in the mood to write or feeling inspired. He gets up and goes to work. It sounds simple, but it's very easy to not notice you're doing this.
At this stage it's important to say there's nothing wrong with grabbing your camera only when you feel inspired to take some photographs; if I recall correctly back when I was just taking pictures for fun, there were few sweeter feelings! This sort of philosophy towards your craft only becomes a problem in any of the following three ways: you want to transition from amateur to professional, you want to want to be the best photographer you can be, or you're frustrated that you are stagnating. If any of those three apply to you and you still only practice your craft when you feel like it, you're more likely than not doomed to remain in that state.
So, practically speaking, how can a hobbyist looking to improve, shoot when they don't feel like it? After all, professionals are given projects and jobs to complete by certain deadlines, so it's hard for them to shoot only when they feel like. Amateurs however, are doing it for fun and tend not to have projects with deadlines. Well, the answer is as simple as you can imagine: set or find projects. There are, for all intents and purposes, unlimited themes, challenges, and ideas for setting yourself a project. Then just add a deadline and you're set. Alternatively, start entering online photography competitions. We have one here at Fstoppers, but there are plenty dotted about all over the place. Forcing yourself to practice your craft outside of periods of inspiration or desire will raise your lowest standard of work as well as your average.
But this problem doesn't just affect amateurs. It can affect professionals in periods of jadedness or even at times of success. How it affects us in times of jadedness is again rather obvious. You get stuck in a routine and you allow enough time to shoot for jobs when you feel like it and still easily make deadline. The one I didn't expect is the latter: during times of success. When you're doing well, it's hard to keep pushing yourself to work on your craft everyday. When you're struggling, you'll be full of motivation to improve, but when you're getting the sort of clients you want and your work does well, that motivation can disappear.
Whether you're an amateur or professional, it's always worth checking that you're "turning up to work". Waiting for inspiration to strike is a luxury only amateurs can indulge.