Randolph veto's his mother's plan of marrying Sam, the sweetheart of her youth. After she is widowed by Vicar Twycott's death, she encounters Sam one day as if by chance, though he willingly confesses to intentionally looking for her as he had heard that she lived along that road:
'I can't come down easily, Sam, or I would!' she said. 'Did you know I lived here?'
'Well, Mrs. Twycott, I knew you lived along here somewhere. I have often looked out for 'ee.'
As they become reacquainted over time, Sam declares in his modest way that he still loves her and longs to have her as his wife--they would run a shop together in their village of Gaymead. Sophy admits that she would dearly like to accept but that her son, who "seems to belong so little to [her] personally," would have to be "informed." She secretly fears this because she doubts Randolph would approve and more greatly doubts she could "defy him" and marry against his protests.
This is where the son's veto begins. When she tells him that she plans to marry a man from her village who is not "what [he] would call a gentleman," Randolph "burst into passionate tears." Sophy cannot defy him and go against his desires--he knows that having a mother married to a village peasant shop keeper would ruin his standing in society and his chances for the kind of success he is aiming at. His passion of tears is his first veto (veto: the right to reject a decision or plan made by another) by which he rejects his mother's desires.
As the years passed, and Randolph held more psychological and social power over his mother, no matter how often she brought up the idea that now she could surely marry Sam quietly, Randolph unceremoniously and pitilessly rejected the idea and forbade her do so. He thus vetoed her desire to marry Sam literally until her dying day.
The primary reason he vetoes her like this is that he is already humiliated and embarrassed by his mother's low social and educational status and correctly speculates that should she marry a villager of her same social, economic, and educational status, he would lose all prestige in society. He would thus be cut from working and socializing in the choicest and highest circles of society--circles he was introduced into as a schoolboy because of his father's care in educating him at the best school and university.
[W]hen he did [speak] it was to say sternly at her ...: 'I am ashamed of you! It will ruin me! A miserable boor! a churl! a clown! It will degrade me in the eyes of all the gentlemen of England!'