Why? Because the more you enjoy what you do for a living, the more time, energy, and resources you will invest; and the more invested you are, the more skilled you can become and, therefore, the higher the external and internal rewards can be.
Think about it. Most likely, the professionally successful (and fulfilled) people that you know of don’t just “like” certain aspects of what they do for a living; they have found what they are truly passionate about.
And how did they find their passion? Well, even if they weren’t entirely aware of it, they leveraged their emotions.