Because you each love someone the other loves.
When a parent marries in later life, many adult children do not see themselves as stepchildren or think of their parent’s new spouse as a “stepparent.” Technically both are true but practically the adult child doesn’t need another parent figure in their life. The answer: relationship. Now that’ll happen when both the adult stepchild and stepparent open themselves to being friendly. Gradually build your relationship. Because you each love someone the other loves, strive to love each other.