1. Death. This is probably the number one reason to allow people to break a lease. Whether the death is that of your tenant, one of your tenants, or someone close to your tenants, death is something that really changes the game for everyone involved in that life. Sometimes people lose a family member and need to travel to take care of the estate or perhaps they have been left a different housing arrangement. Always have a plan about when death hits at least close to home for those on your lease.
2. Loss of Work. Sometimes, losing your job means the need to cut back. Unfortunately, this does sometimes mean the need to find a cheaper living situation. If your tenant has lost their job and can no longer afford the terms of your lease, you may be able to help out by allowing the lease to be broken and not continuing to charge them something they cannot afford.
3. Severe Illness of Injury. When someone gets sick or hurt, everything can change. Sometimes they need to stop working. Other times, medical bills can pile up to the point that any amount of income that doesn't go towards food needs to go to that so they can continue to be treated. If your tenant or someone close to your tenant has a situation like this, then you may see fit to break the lease you have. If your tenant needs to move to care for someone who has become disabled, then breaking the lease may be the best thing for all involved.
While it is always painful to break a lease, sometimes it is the best thing that you can do for your tenant. After all, if they cannot continue to pay their rent and are open about it, at least you know that they are responsible. You never know, perhaps if the issue that caused the lease to be broken is resolved, they may just come back.