http://www.ccel.org/ccel/mcgarvey/gospels.vii.xlv.html
23 And after he had sent the multitudes away, he went up { b departed} a into the mountain apart to pray: and when even was come, he was there alone. [The news of John's assassination was calculated to exasperate him in the highest degree, and also to deeply distress him. He needed the benefits of prayer to keep down resentment, and to prevent despondency. For this he started away as soon as he heard the news, but the people prevented him till night.]
The commentary reminds me of Jesus’ humanity. John’s death weighed upon his heart, and he probably wished to grieve. It’s so hard to come to grips with the idea of the divine and the human natures wrapped up in one person, and I need to remember that Jesus could feel the human emotions of sadness and frustration.