תהילים מב
"As the deer longs for brooks of water so my soul longs for you Hashem"
A deer goes to the water and he sees his reflection. He thinks he is seeing a female deer. So he has a תאוה to be with the female but it's "NOT happening". The way desire works is that when unrequited it intensifies. So the deer has stronger and stronger feelings of passion for the female.
Our soul's desire to be close to Hashem is similar. It keeps getting stronger and stronger because in this world its desire to get close to Hashem is never fully satisfied. Our souls are חולת אהבה - lovesick.
You don't feel it sometimes? Just a matter of getting closer to your soul...
