I couldn't say it any better than this, so I'll just quote it. John Stott is right, as usual.
Incentives are important in every sphere. Being rational
human beings, we need to know not only what we should be
doing, but why we should be doing it. And motivation for
mission is especially important, not least in our day in
which the comparative study of religions has led many to
deny finality and uniqueness to Jesus Christ and to reject
the very concept of evangelizing and converting people.
How then, in the face of growing opposition to it, can
Christians justify the continuance of world evangelization?
The commonest answer is to point to the Great Commission,
and indeed obedience to it provides a strong stimulus.
Compassion is higher than obedience, however, namely love
for people who do not know Jesus Christ, and who on that
account are alienated, disorientated, and indeed lost.
But the highest incentive of all is zeal or jealousy for the
glory of Jesus Christ. God has promoted him to the supreme
place of honour, in order that every knee and tongue should
acknowledge his lordship.
Whenever he is denied his rightful place in people's lives, therefore, we should feel inwardly wounded, and jealous for his name.
--From "The Message of Acts" (The Bible Speaks Today series: Leicester: IVP, 1990), p. 279.