A person decides to live his Christian life when he decides to do God's will instead of his own will (see Mt 26:39). Solomon prefigured this when he sought God's heart and will rather than his own well-being, prosperity, or victory (1 Kgs 3:11ff).
Once we decide to do God's will and get involved in God's life, we, as human beings, naturally and rightly develop a sense of belonging and ownership. Our service to the Lord becomes not just God's will but our will (see Acts 15:28). Nevertheless, we can take too much ownership of doing God's will and fall into the subtle temptation of making God's will primarily our will and secondarily His will. At this point, although we are still technically doing God's will, we are doing it more for ourselves than for Him. We are doing God's will with the wrong motivation.
To check this, the Lord brings interruptions into our lives. Will we drop God's will to do something else, which suddenly is manifested as His will? When we are owning God's will and appropriating it to ourselves, we have great difficulty making the transition when God interrupts us, but that transition is relatively painless for those doing God's will primarily because it's His will. For example, Jesus was doing the Father's will when He took His apostles aside to rest and pray (Mk 6:31), but the Father had additional plans. Jesus made the transition smoothly and taught the masses at great length (Mk 6:34).
Check your transitions when presented with godly distractions. Are you doing God's will primarily because it's His or primarily because it's yours?