Ballet flats are not the best shoes for your feet!  Because they are so flat, they lack any arch support which can cause your arch to flatten arch.  this lack of arch support over a period of time can cause arch pain and heel pain leading to plantar fasciitis, and Achilles tendinitis.  You can give yourself arch support by placing custom orthotics in the ballet shoes to support the foot, and this will prevent pain.  So although those ballet flats are fashionable, they may not be the best for your feet!