Recently several people I know have died and the survivors who loved them are dealing with grief. That certainly substantially limits their major life activities, so perhaps grief should be considered a disability.
I'm not suggesting that people who have lost a loved one should be entitled to use handicapped parking spaces, but they should certainly be given allowances for being unable to accomplish things they usually can do. For example, employers should be understanding when the work of someone experiencing grief is not up to par and insurance companies should be willing to pay for counseling when it's needed.
Fortunately grief usually becomes less severe with time as people work through the four stages and eventually the effect on major life activities will be much milder.