Control, projection or possessiveness could make this friendship a difficult one. You stir up feelings in each other and may trigger old psychological patterns. This relationship may be less about mutual support and growth and more about dependency and the acting out of subconscious patterns.
This is particularly true if one of you has any type of authority, superiority or seniority over the other. You may both sense a deep, powerful but obscure struggle that pits your individuality against your friend's needs or desires. You may end up feeling unseen, unheard and unsupported on a deep level. These could lead to resentments that will continue to store up over time unless they are addressed. However, if you can work through this aspect of your relationship, the two of you can become lifelong friends with an incredibly intimate bond.