Bodies are weird. If the sight and sound of someone cracking their neck sets you off, images of cadaver dissections gross you out, you swoon at the sight of blood, or any other bodily fluids and functions make you want to vomit -- this might not be the best career for you.
You lack reliable transportation.
Some massage therapy programs are hours-based, so every minute has to be accounted for. You need to be able to get to school on time; if not, how are you going to show up to your future jobs on time? You can't rely on your clients to be understanding 100% of the time.
You're unable or unwilling to practice outside of school -- including asking friends, family, and acquaintances if they would like a free session.
If you can't give the work away for free, how are you going to market it and get paid for it? It also comes down to the quantity of sessions you give -- the quality will improve later. Sleeping with a textbook under your pillow will not, in fact, increase your knowledge by osmosis. You have to physically put your hands on people to develop your skills.
You have significant physical disadvantages or health concerns that make giving and receiving massage difficult and/or inadvisable.
Giving a massage is physically demanding. You need to be able to stand, lunge, lean, squat, and lift -- and a lot of this movement comes from having strength in your knees, thighs, and core. If you have cardiovascular, nervous system, or liver or kidney disorders, receiving massage may be contraindicated (that is, "a bad idea") for you.
You're unable or unwilling to clean, follow rules and regulations, and/or otherwise be professional.
Your caregivers are not going to be following behind you during school or at work to make sure your space is clean. During your training, you will have opportunities to learn skills such as laundry, sweeping, vacuuming, and keeping a bathroom and other surfaces clean. Sanitation, infection control, and universal precautions are especially important considering our close-contact with complete strangers. You will be given specific expectations and guidelines to abide by that are often meant to prepare you for work after graduation; some rules and regulations are state-mandated, while others are in place to qualify you and your school to receive federal financial aid. Finally, you're going to need to be polite, civil, and a generally pleasant individual to work with. These are skills you will be able to develop and practice during school, but it's preferrable that you have some familiarity with them beforehand.