1. Regain the relationship 2. Regain control and/or 3. Get revenge in some way
Some people bolster their self-esteem by dominating, intimidating, possessing, or manipulating partners. Controlling or affecting a partner gives them some feelings of power or uniqueness in a world where they generally feel powerless or unimportant. This kind of person, after becoming an "ex", sometimes becomes a stalker to try to continue fulfilling these inappropriate psychological needs.
In cases of revenge stalking, the stalker believes they were "wronged" in some way and they cannot "grow up" and move on with their lives.