Many. Genetic, environmental, trauma, medications, eye disease. Can be an alignment problem due to many causes or an uncorrected / poor vision problem.
Multiple reasons. Amblyopia can develop due to focusing differences between the eyes, crossing of the eyes (stabismus), occlusion from things like cataracts. If these problems are not identified and treated then vision loss may become permanent.
Differences. Amblyopia is a progressive event that will develop if the eyes present the brain with 2 different images of the same object. If one eye develops weaker vision, the brain will eventually prefer the stronger. If alignment changes and gives a double image, the brain selects one. If you fix the weakness or re-align the images you may be able to stop the process. If not, the vision loss can be permanent.