According his own self-conception he had little alternatives.
The re-conquering of the west was his duty - he was the Roman emperor.
Sourrounded by reliable officers, generals, statesmen, famous senators and an army he had the tools to "try" this experiment.
Terms and conditions were propitious.
But even Justinian was human - a person - and no machine.
His claim to rule over the west was legal and finally successful, even if some old roman provinces were still missing (e.g. gaul or Britannia).
A Roman emperor was once more the ruler over an united Imperium Romanum.