How can I respond to my friend's question about why I stopped seeing him and didn't visit him throughout my university studies?

Friendship and love among Muslims are a great objective, and one should only associate with the good and the pious, for the Prophet, peace and blessings be upon him, said: "A man is upon the religion of his friend, so let one of you look at whom he befriends." Love for the sake of Allah is what endures, for Allah Almighty says: "Close friends on that Day will be enemies to each other, except for the righteous." If your friend is someone whose companionship you approve of, then communicate with him as much as you can. If he reproaches you for not visiting, apologize by citing your study commitments.