Anyone who has an interest in your estate may contest your Will, however he or she would need to prove one of the following:
a) That you were not of sound mind (that you lacked sufficient mental capacity when you signed your will)
b) That you were subject to undue influence or were under duress when you signed your Will
c) Fraud.
Also, if the language in your Will was ambiguous or incomplete, the court could interpret the Will in a manner you did not intend. Having an attorney prepare your Will can minimize the likelihood of success of someone contesting your Will.