Lady F
It is a portrait of a young boy of means. In other words, his family had money. The artist wanted to show that, which is why he is wearing such frilly clothing. The painter also wanted to show his skill as a landscape artist, which is why the boy is placed outside. The background is darker so as not to take away from the subject matter - the boy. The boy's family would have paid the artist to paint his portrait for the family gallery. The boy's demeanor lends to the fact that he felt he was an important person.

spicy iguana
Thomas had a rival (I forget his name) who stated that you can only have blue in the background of a painting, not in the main subject. He made that painting just to spite his rival, and received quite a bit of attention for it. Because of that painting they didn't speak to each other for years until one of them was on his deathbed.