Full Idea
Because will moves itself by deliberation - a kind of investigation which doesn't prove some one way correct but examines the alternatives - will doesn't compel itself to will.
Gist of Idea
Because the will moves by examining alternatives, it doesn't compel itself to will
Source
Thomas Aquinas (Quaestiones Disputatae de Malo [1271], Q6.reply)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.178