Being protective of a friend is love. Being possessive of a friend is fear.

The two look similar from outside and run on opposite engines. Protective is about what happens to them, and it is happy when they have more people looking out for them. Possessive is about what happens to you, and it tightens when somebody else gets close. One wants your friend to have a good life, and the other wants to be the main part of it.

Watch your own reaction when a friend makes a new friend. If it feels like loss, that is worth being honest with yourself about rather than acting on. Say something warm about the new person and mean it.