Cap. III Sol. Rat. 5: loving and electing make worthier; Colossians 1:12
The reason: by loving and electing, God makes those he loves and elects worthier and better. Human choice cannot create disparity of worth in what it chooses, so it must aim at the object where greater dignity is already assumed. For divine election's justice and wisdom it is enough that, though all people are unworthy of this electing gift, God by electing makes those he elects to glory fit and worthy ahead of others to share the saints' lot in light (Colossians 1:12).
Latin
Hujus autem rei hanc rationem afferre licet, Quia Deus diligendo et eligendo facit eos digniores et meliores, quos diligit et eligit. Humana electio, quae facere non potest disparitatem dignitatis in rebus electis, tenetur tendere in illud objectum in quo praesupponitur major dignitas: sed ad Divinae electionis justitiam et sapientiam statuendam sufficit quod cum omnes homines sint indignissimi hoc electionis beneficio, Deus eligendo facit illos quos ad gloriam eligit prae aliis dignos et idoneos ad participandam sortem sanctorum in lumine, Coloss. i. 12.