Love for God is the only sufficient motive for evangelism. Self-love will give way to self-centeredness; love for the lost will fail with those whom we cannot love, and when difficulties seem unsurmountable, only a deep love for God will keep us following His way, declaring His Gospel, when human resources fail. Only our love for God, and more important, His love for us - will keep us from the dangers which beset us. When the desire for popularity with men, or for success in human terms, tempts us to water down the Gospel, to make it palatable, then only if we love God will we stand fast by His truth and His ways.
I would definitely agree - loving God is the only sufficient motivation for witnessing. Too often, my evangelism (or lets be honest - my intended-but-not-carried out evangelism) is fueled by guilt - "I'm a Christian, I'm obligated to witness to this person." Or, "Wow, such and such a person witnesses all the time. I want to measure up to what they're doing."
Thinking like that does not supply the courage and power we need to be faithful witnesses, and it doesn't honor God.
I'm praying right now that God would increase my passion for the Gospel - that I would be so overwhelmed with thankfulness and awe by Christ's work on the Cross that I cannot help but open my mouth to share the good news with others.