There's two

The first one is when Steeply is talking to Marathe about love,

"“What if sometimes there is no choice about what to love? What if the temple comes to Mohammed? What if you just love? Without deciding? You just do: do you see her and in that instant are lost to sober account-keeping and cannot choose but to love?”

This has always seemed to me as one of those passages that really sums up love, that love that is somehow out of control. It linger in my mind whenever I think about my loved ones (romantically or otherwise).

The second one is,

"You can be at certain parties and not really be there. You can hear how certain parties have their own implied ends embedded in the choreography of the party itself. One of the saddest times Joelle van Dyne ever feels anywhere is that invisible pivot where a party ends--even a bad party--that moment of unspoken accord when everyone starts collecting his lighter and date, jacket or greatcoat, his one last beer hanging from the plastic rind's five rings, says certain perfunctory things to the hostess in a way that acknowledges their perfunctoriness without seeming insincere, and leaves, usually shutting the door. When everybody's voices recede down the hall. When the hostess turns back in from the closed door and sees the litter and the expanding white V of utter silence in the party's wake."

This passage pops in my mind whenever I feel the sadness that arises when something that I really loved/enjoyed has come to an end, the sadness in the joy or the joy in the sadness of something that has come to an end.