Learn English – Fitting interjection for “that’s nothing”

It's true that when that feeling of longing and desire strikes, words just aren't adequate.

A drawn out mmmmmmmm-Mmmmmmmmm!" should be understood to be an expression of approval (with the emphasis on the 2nd Mmmmmmmm).

OR

A 2-note whistle, like a wolf call but kind of done under one's breath also conveys the same meaning of "oh boy!".

If those suggestions aren't enough, a long, audible inhale followed by an equally long and audible exhale have the same meaning - if performed as part of the same interaction with the friend and the object of desire.

Those were the days fits for all contexts.

Usually with "Ah" in the beginning of the expression. Sometimes, "Man" is used in the beginning also.

something you say that means life was better at the time in the past that you are talking about:

We were young and madly in love. Ah, those were the days!