United signed the former Luton Town midfielder on a free transfer earlier this week, as the 25-year-old rejected a move to Nottingham Forest.
Spring told the official Leeds website:"It was a bit of a shock because I was lined up to go to Forest and I was pleased with that. Then a couple of days later Leeds came in with an offer.
"I had a decision to make but as soon as I saw Elland Road and thought about the atmosphere of 35,000 fans at every home game my mind was made up.
"I feel great about it. It's all been a bit of a whirlwind but it's a great opportunity."