Have you ever looked up into the sky on a cloudless and starry night?  Did you see how the heavens stretched on and on like they would never end?  That is exactly how much love and faithfulness for us the Father has in store.

When David wrote this Psalm he was literally waiting for king Saul's men to track him down like Dog the bounty hunter, but the only difference was they weren't taking him in alive, they were out to kill him.  Now since David knew that the LORD never forsakes those who belong to Him, all he had to do was call out for GOD to save him and it would be so.  How did David know that GOD'S faithfulness stretched so far?  Personal experience.  David had needed to call on the LORD many times before, from guarding his fathers sheep from wolves and bears, to facing down Goliath.

GOD never ran off and left him out in the cold before, so why should He start now?  Just as GOD'S faithfulness was abounding to David, so it is for you and I, all we have to do is seek GOD, He is willing to prove His faithfulness to us without end.