A lot of people think learning the harmonica is easy but this is not the case for some. It really depends on each individual and whether they have an affinity to kinaesthetic learning.
1. It is small and you can put it in your pocket and go!
Yes, it is one the smallest instruments to choose from and it certainly is easier than carrying around an acoustic double bass or a grand piano. You can go for a walk and play to the rhythm of your steps. In fact, this was how I got started (besides playing it in the shower). The size makes it handy to take with you on holidays and keep it on you at all times so you can play it whenever you get the urge.
2. It is a wind instrument that can play chords as well as single notes.
Wow! Usually wind instruments can only play single notes. This opens up a plethora of possible combinations and rhythms in order express yourself as a soloist or part of a group.
3. It is invisible!
I can't think of another instrument where it is unseen to the audience. You can't really tell how a harmonica player is achieving their sound. This creates a attractive mystery to the instrument and may be a reason to want to learn the secret business of harp playing. It is also a good reason to have a teacher to help unravel the technical aspects of learning this clever little instrument.
4. You can play more than just the Blues.
If you so desire, you can play Classical Music, Jazz, Indian Music, Bluegrass, Pop, Swing, Irish Music, Klezmer, Middle Eastern Music, and more, on a standard 10 hole diatonic harmonica. The Blues is probably the most popular reason that most people want to learn the harmonica but the possibilities are endless.
If you are thinking of learning harmonica, keep in mind that just because it is small, does not mean that is easier than other instruments. A student needs to develop their sensitivity to small movements and muscle memory. It is definitely a fun instrument once you get going and is a great one for jamming and improvising.