Set up situations where your dog will be tempted to run to their favorite obstacle. This will probably take time and patience, but the rewards will be worth the effort. Run the dog through the set-ups just a few times, then do something else for awhile. Some dogs pull in to their handler easier and others push away easier, and your handling strategy for a difficult sequence in a course must take that into consideration.
Obedience needs to be trained outside of the agility ring first.
As far as the relationship component, obviously we want our dogs to obey not because we force them to, but because they want to. Using a leash or longe line, set up situations to illustrate to your dog that he must respond immediately when you call him no matter what he is doing- whether he is laying in the corner, sniffing under a tree, chasing a ball or heading for an obstacle. This is a quick way to take the enthusiasm out of the sport for the dog, particularly a soft dog. If your dog learns not to trust your body language, a major component of the team is lost. What may seem to be the dog's fault at the time is often revealed to be the handler's fault when you watch the tape.