From a simple Google search:

Thomas had eight siblings, and was the youngest child.

While the rest of the family's sons pursued military careers,[12] the family intended for Thomas to follow his uncle into the abbacy;[13] this would have been a normal career path for a younger son of southern Italian nobility.[14]

And biography.com asserts,

Before St. Thomas Aquinas was born, a holy hermit shared a prediction with his mother, foretelling that her son would enter the Order of Friars Preachers, become a great learner and achieve unequaled sanctity.

You're trying to make this about him as an individual, when in fact it seems like a combination of family needs and inheritance customs/law that even a hermit could predict in advance.