You won't understand my reasons for leaving, but you deserve at least to hear them. Maybe you'll reread them eventually and they'll make sense.
I am not a mage. My talents in magic, by your own contemptuous admission, are weak. My skills lie elsewhere, and I plan to put them to use. I will never be the prodigal mage you hoped for, but you have never heard me sneak in or out of the house, nor do you likely know that I overheard your comments to your wizard friends over dinner. I know you think little of me, and wish I had been born otherwise, but I have not, and I'll spare you the further burden of my company.
I will send money home, a portion of that I make. Whether you find it proof that I am doing well for myself or simply find it useful to put food on your table matters not to me. Consider it slow repayment for the years you have dealt with the burden of an underachieving son.
I love you dearly, Father, but I cannot abide your company. Live well.