John Chrysostom

For what purpose doth He go up into the mountain? To teach us, that loneliness and retirement is good, when we are to pray to God. With this view, you see, He is continually withdrawing into the wilderness, and there often spends the whole night in prayer, teaching us earnestly to seek such quietness in our prayers, as the time and place may confer. For the wilderness is the mother of quiet; it is a calm and a harbor, delivering us from all turmoils. …

Homily on the Gospel of Matthew 50 ↗

Theophylact of Ohrid

He went up on the mountain to show that we should pray in an undistracted manner; everything He did was for our sake as He Himself had no need of prayer. He prayed on into the evening, teaching us not to cease praying after a short time, and also to pray especially at night, for it is very quiet then. He permits the disciples to be caught in a storm, so that they might learn to endure trials bravely and that they might know His power. The boat was out in the very middle of the sea so that their fear would be greater.

Commentary on Matthew ↗

Jerome

And when the crowd was dismissed, he went up the mountain alone to pray. But when evening came, he was there alone. If Peter, James, and John, who had seen the glory of his transfiguration, had been with him, perhaps they would have gone up the mountain with him. But the crowd cannot follow to the heights, unless he teaches them by the sea on the shore and feeds them in the desert. …

Commentary on Matthew ↗

Hilary of Poitiers

Or, that He is alone in the evening, signifies His sorrow at the time of His passion, when the rest were scattered from Him in fear.

Catena Aurea by Aquinas ↗

Thomas Aquinas

And having dismissed the crowd, he went up into the mountain alone to pray. There follows the danger, and the danger is shown from the time, from the place, and from the wind. And first the absence of Christ is presented, because while he was with the disciples, he went up into the mountain alone to pray. …

Commentary on Matthew ↗