One of the biggest issues I see in student's painting is not understanding the subject they are painting. Instead, they try to simply paint what they see in their reference. It's understandable that we want to jump right into the painting and enjoy the process. However, to paint a believable watercolor painting requires a good amount of thinking. I made that mistake sometimes as well. I try to just blatantly copy what I see instead of taking a brief moment analyze and understanding what I am looking at. I'm convinced that a good painting demonstrates and show the artists' understanding of the subject matter. How does the object sitting ina 3d space? How does the lighting work with the scenery? What's the surface material of the object you are painting? These are all very important questions you should be asking yourself before and during the painting.
Your painting looks more alive when you paint what you understand
Imagine you are trying to retell and explain a story you've just heard. However, you didn't really get the story yourself. It could be due to you missed part of the story, or you weren't really paying attention. It will be a struggle to retell the story to someone else because you cannot retell something you haven't heard!(unless you make things up). That is what happens when you try to second guess in your paint. An experienced painter might be able to make things up, spice the story up a bit and tell a different story. But the things they make up are still based on their understanding of the real world. They have a rich visual library to pull from.
Only through understanding can you paint a believable painting. And only through understanding can you produce a painting that looks alive. All the painting techniques and skills are very important, but your understanding of the subject will make your painting stand out and gives it characters.