One of the most common reasons a lease is broken is because of a change in employment. Many times a tenant will lose their job and it hardly matters if it was their fault or not. Other times they are transferred and cannot say no to the relocation for a multitude of reasons. While this is an unfortunate situation for both of you, the tenant obviously cannot continue to make payments if they are unemployed and they should not have to pay rent on two different locations if they are only living in one. This is especially true of those in the military, as they can be relocated at a moment's notice.
Extreme circumstances are another time that you should consider allowing your tenant to break their lease. If there was a death in the family, many people will want to move to either get away from the memories or get closer to those they still have left. Sometimes people need to move in with ageing parents to help take care of them. Disabilities are another circumstance that may be unforeseen and need accommodations that you cannot provide. Sometimes horrible things happen in our lives and your tenants are no different in that regard.
While some landlords are adamantly against breaking leases, there are certain situations in which it is understandable why a tenant would want to get out of the contract early. Always remember that your tenant is human just like you are and sometimes these things unfortunately need to happen.