But why did He not withdraw His body from the Jews, and so guard its immortality? (1) It became Him not to inflict death on Himself, and yet not to shun it. (2) He came to receive death as the due of others, therefore it should come to Him from without. (3) His death must be certain, to guarantee the truth of His Resurrection. Also, He could not die from infirmity, lest He should be mocked in His healing of others
But it were better, one might say, to have hidden from the designs of the Jews, that He might guard His body altogether from death. Now let such an one be told that this too was unbefitting the Lord. For as it was not fitting for the Word of God, being the Life, to inflict death Himself on His own body, so neither was it suitable to fly from death offered by others, but rather to follow it up unto destruction, for which reason He naturally neither laid aside His body of His own accord, nor, again, fled from the Jews when they took counsel against Him. 2.
But this did not show weakness on the Word's part, but, on the contrary, showed Him to be Saviour and Life; in that He both awaited death to destroy it, and hasted to accomplish the death offered Him for the salvation of all. 3. And besides, the Saviour came to accomplish not His own death, but the death of men; whence He did not lay aside His body by a death of His own — for He was Life and had none— but received that death which came from men, in order perfectly to do away with this when it met Him in His own body. 4.
Again, from the following also one might see the reasonableness of the Lord's body meeting this end. The Lord was especially concerned for the resurrection of the body which He was set to accomplish. For what He was to do was to manifest it as a monument of victory over death, and to assure all of His having effected the blotting out of corruption, and of the incorruption of their bodies from thenceforward; as a gage of which and a proof of the resurrection in store for all, He has preserved His own body incorrupt. 5.
If, then, once more, His body had fallen sick, and the word had been sundered from it in the sight of all, it would have been unbecoming that He who healed the diseases of others should suffer His own instrument to waste in sickness. For how could His driving out the diseases of others have been believed in if His own temple fell sick in Him? For either He had been mocked as unable to drive away diseases, or if He could, but did not, He would be thought insensible toward others also.
Source: On the Incarnation of the Word (New Advent)