I need a reason to keep learning this language.
After completing the tree (which was my main goal) I have little motivation to continue learning it. I like the language but I like German more so I just wonder if I'm wasting my time on something barely anyone speaks.
Why do people learn languages?
For some, it's just a means to an end in itself.
For some, it's to read great literature. (Or just to read other things that are only in that language.) If that describes you, buy some Esperanto books from a libroservo - for example, the UEA (Universal Esperanto Association) has a webshop, as do many other regional Esperanto associations. You may wish to choose one in your country or in one nearby - for example, ELNA / Esperanto-USA if that's where you live: www.esperanto-usa.org/retbutiko/
For some, it's a way to talk with their friends. To do that, you may need to meet some first... go to a local Esperanto event or to something like the yearly NASK in America or SES in Europe.
If you need a reason to continue learning it and don't have one - why not drop it? Esperanto won't feel bad. You don't have to learn it or keep using it once you've reached a certain stage.
If it fills no need in your life, then feel free to move on.
I think having an Esperanto Twitter account is a good way to use and have fun with the language. You can tweet to people from all over the world and they will be happy to respond to you (you won't get the same reaction in English). Bonus: People can correct your tweets with private messages if you ask them to!
Esperanto is easy. Keep it up and you'll achieve real, useful fluency. That to me is a good enough reason to continue: to actually be fluent in another language. It takes a fraction of the time to achieve fluency in Esperanto as other languages. That means you could keep up with Esperanto AND learn German if you wanted.
Esperanto has many good reasons to continue: 1, You can be perfect with a language within 1-1,5 year and this is a great success for You, for any other languge it would take 8-10 years 2, You can understand roman languages much better and vica versa, I used Esperanto in Italy for example and it was working, people understood me, with french it is even more the case 3, You will know a very cool language, which is now growing and can have a bright future (if only translations of TV shows in internet will appear in mass, this language could continue with grow) 4, This is the language of the translations, and think this can add to this language a competitive edge, no other languages can be used for translations so simple and precise, than this one 5, I am learning additionaly, because seriously I think, that this language has a big future (I don't understand why roman language countries are not teaching and using it in mass, they could use it extremely quickly for inter roman language communication for example)
You can find people to talk to in Esperanto on the internet, but if your main goal was to finish the tree and you have no desire to speak it with anyone then maybe you are wasting your time continuing to learn it. I'm still going through the Esperanto tree and I'm not sure if I will continue with Esperanto once I'm done since I'm mainly doing it as a hobby. It all depends on what your motivation is.