A lot of times you'll hear non-Calvinists ask something like "if everything is ordained, why do anything?" What they mean is, if God predestines everything, why exert any energy in either obedience or evangelism? There is a tinge of legalism beneath this question sometimes. What they are sometimes getting at is, that the Calvinist should not feel like he "has" to do anything in light of the fact that his position is secured with God. But the Calvinist never does anything because he "has" to. He obeys God because his nature is transformed in such a way that he naturally desires to obey God. Joe Morecraft III put this really well. Why do I evangelize? 'cause I like it. We have a new nature and we naturally "like" to do this sort of thing. We don't need a reason to do it other than that we enjoy obeying God to his glory.
Ask the Arminian why he likes ice cream. Why bother pursuing ice cream if all history is already fixed? God knows what's going to happen, right? Well that means all history is certain. Why do anything? Why eat ice cream? Because we like it(I like ice cream too)! We don't need metaphysical argumentation or speculation to justify doing what we like. We just happen to enjoy obeying God once he has transformed and renewed our natures. Just as it was the food and drink of him to do the will of God(Jhn. 4:34), it is our "ice cream"(I say this with reverence) to do the will of our God and Father.
"Doing wickedness is like sport to a fool, And so is wisdom to a man of understanding"(Prov. 10:23).