This letter seems to have been written to encourage Timothy to keep going, as there is a theme of endurance throughout. This might also be because the writer himself needs to practice endurance. The dating suggests that this could have been the last letter that Paul wrote before his death. If so, with his last letter he chose to encourage and spur on a young person. 

This letter encourages us to push through, even when life seems tough. There are some great themes here about digging deep and keeping going, even when you don't feel like it.