The reality action you refer to seems to be:

I as human enactor of results am evaluating a relationship–between myself and someone else–and I find it to be of importance or significant positive effect for me; it is therefore something worth investing time and emotion it, continuing, perhaps improving or increasing.

So for the words:

“I value this relationship” one could substitute other meanings, shades of the same reality reference to the situation described.

I enjoy being your (choose the noun).

I like what we have together.

I want us to continue to be (supply noun plural).

I feel pleasure that we are (supply noun plural).

I place value on our (supply noun).

It’s important to me that our relationship continue.

One cannot as an English speaker replace “value” with another or any other term since it is relates to categorical positive and negative understanding and/or feelings toward something (without one stressing any specified amount of value). The inference is of importance, that pleasurability, liking, enjoying, acknowledging and not losing that value are implied but not in degree. That’s what makes it a category–a designation of a type f thing in reality with amounts and parameters specificable toward something outside of itself only in specific contexts.

Your sentence is a lot like,

“I enjoy attending sports events”.

Substitutions here would be

I like

I enjoy

I value

I feel pleasure in

I want to continue

It’s important for me to…

See?

Have a great day.

this friendship/relationship we have.

I value your friendship.

This friendship means a lot to me….it is of value to me.

OR

You have a special place in my heart.

OR

You are a very special person to me.

What you and I have is precious to me.

Thank you for being my friend.

This relationship is worth it to me.

This affiliation is worth it to me.

?