There are many reasons to learn Esperanto. It's easy, expressive, neutral, fun and can help you in many respects.

Easy

Who likes to learn irregularities, unintuitive idioms and intolerable amounts of grammar? These are evils you have to put up with when learning a natural language, but shouldn't be part of the ideal easy language. (Learn how easy Esperanto is.)

Expressive

Using Esperanto, you can express yourself just as well as you can in your native tongue, and in some cases you will find that Esperanto allows you to express something better. This is due to the 100% logical word creation system, which allows you to bend the language to your will. The same system is also responsible for decreasing your vocabulary workload by ten times. (See some examples of this system in action.)

Neutral

Esperanto doesn't belong to any one religious, political or ideological belief, beyond the idea that all people should live together in peace and tolerance. On the level that Zamenhof originally intended, this means that there won't be language domination, for example Arabs won't have to speak Hebrew and Native American languages won't be abandoned in favor of English. On a level that's more relevant today, it means that you won't have to feel awkward or inferior when talking to foreign people, as you might feel awkward when speaking Japanese to somebody from Japan, because with Esperanto you're on equal footing; nobody has the advantage of knowing the language perfectly.

Fun

Apart from the fun of learning Esperanto (read the testimonials), Esperanto is also fun to use. At home, you can probably use it as a secret language for you and your friends, and elsewhere it becomes the language of friendship. There is a real spirit of community among Esperanto speakers everywhere. Other languages lack this spirit of community, because speaking another language is (for the native speakers and a lot of non-native speakers) just a matter of upbringing, not one of personal choice. Learning Esperanto is always a personal choice and thus it links those who made it. This also means that most Esperanto speakers have interests and mindsets in common, for example open-mindedness, thinking independently, high level of interest in other cultures, interest in languages...

Great for traveling

Wherever you go, if you speak Esperanto you can look up the local Esperanto speakers, talk to them and meet them, maybe get to stay for free at their homes (-> Pasporta Servo) but at least have people who will gladly share with you their culture - and interpret for you and make sure you get local prices if you don't speak the local language. If you want to travel to more than one language region, it is more valuable than learning any single language, especially if that language is so difficult that you can hardly express yourself after studying it for a year. Spanish may be great for South America, but it won't get you far in Japan.

Great for staying at home

Even when you can't travel, Esperanto allows you to discover foreign cultures and exotic countries. For example, you could act as a host for Esperantists traveling to your place, or you could participate in Esperanto clubs or meetings, where many people from different cultural backgrounds meet. And if you don't want to even leave your armchair, you could enjoy Esperanto music and literature. People from everywhere in the world write original literature in Esperanto. If they don't write themselves, they translate famous national authors, who often don't get translated to English. This way you can enjoy all the world's great literature.

Helps you learn other languages.

According to many independent studies, Esperanto is similarly useful as Latin in this respect, and considerably easier to learn. Esperanto helps you learn other foreign languages in the following ways:

• You will be able to recognize words when learning Romance and Germanic languages
• Your understanding of grammar will improve; for people who don't know any foreign language yet it would actually make sense to first learn about grammar in a language with few rules and no exceptions, because it's less confusing and easier to understand. After that they would find it easier to learn a language with similar rules which applies them only in a haphazard manner.
• Your understanding of logic and of how languages work will improve; the affix system and flexible word combination system broaden your horizon. Esperanto will both force you to look at the heart of things (e. g. if you don't know the word for 'glasses' you could refer to them as 'vidilo' = seeing-tool), and to open your mind to the possibility of new words.

The Ideal!

For all these reasons, Esperanto is really the ideal lingua franca. This has also been recognized by the UNESCO on two occasions. There is no reason for the UN and EU to waste billions on translations; there is no reason to make people worldwide to suffer through English lessons, Spanish lessons, French lessons, and so on if they aren't interested in these languages. However, petty politics and hard economic interests keep nations from accepting what is clearly the best solution to the world's language problems. The change has to come from the people and contributing to this change can make you feel good.

Further reading

If you want to read more about reasons to learn Esperanto, I can recommend the following article: Why does an American use Esperanto? by Joel Brozovsky.