A transaction is an arrow that ends; a relationship is a whirlpool that keeps turning. When a deal dies, the person still gets a message.
I think of relationships as a whirlpool you maintain year in, year out, with no real end in sight. A transaction is an arrow, from the start to closed won or lost. Sometimes the whirlpool spins off an arrow: someone you've stayed close to finally decides to move, or has a problem you can solve. The mistake I've made, and see plenty of people I work with make, is letting the transaction become the only thing that matters. Most business opportunities fall through. What you do afterward is the part people remember.
A while back an investment deal with someone I'd known for years fell through, which is normal. Then they went silent, and I later found out a few others got the same treatment. Years of relationship, wiped out by a few months of pitching and negotiation. It made me look at my own habits, and gave me the rule: the opportunity can end, the person still gets a message.
"This one didn't work out, let's grab a beer."