The Form
We shouldn't be surprised to see a sonnet from any Romantic poet. Literally a sonnet is a "little song." That was the perfect container for the Romantics, who wanted quick, personal reflections on their experience of the world. Keats was especially talented with sonnets; in fact, many critics agree that after Shakespeare, Keats wrote the finest sonnets in the English languages (sorry, John Milton!). Almost all of Keats's sonnets are Petrarchan, though he wrote quite a few Shakespearean sonnets later in his career.
Ambition
Keats wanted to do something great. He wanted to write epic poetry, to be remembered among the greatest poets who ever lived. So this poem about the power he sees in Chapman's poetry fits right in. He believes that being a poet is equal to exploring new worlds and staring off at undiscovered oceans. You can hear his excitement. Later, this excitement will be colored by his fear that he won't live long enough to achieve this fame. His most famous sonnet, "When I have fears that I may cease to be" deals with this more morbid idea, too.