Why did Paul start betting on horses in "The Rocking-Horse Winner"?

Paul starts betting on the horses because he senses a fundamental lack in his house, a missing thing. The house’s whispering of “there must be more money” is a kind of subconscious expression of another lack—his mother’s love. Lawrence writes that the mother had "at the centre of her heart . . . a hard little place that could not feel love, no, not for anybody" that only she and her children understood. This unspoken truth is just one of many unspoken truths that make up Paul’s world—along with the whispering about money, Paul is also driven by the mysterious desire to be “lucky,” or the mysterious feeling of "being sure" about winners. But the story shows that luck is not the same as love; it’s clear at the end of the story that even though he’s won eighty thousand pounds for his mother, it will never be enough to fill the empty place inside her.

Why did Paul start betting on horses in "The Rocking-Horse Winner"?

Paul starts betting on the horses because he wants his mother to love him. He senses she does not truly love him or his siblings, and he knows she constantly needs money. Because he can supernaturally predict the winners of races by riding his rocking horse, he begins to do it obsessively, then gives all the winnings to his mother. However, no matter how much Paul wins, it never seems to be enough for her, so he feels he has to rock harder and harder. Paul is too young to realize he cannot win his mother's love by winning money for her at the races. She has a fundamental hole in her heart, which is her incapacity to love others, and no amount of money will ever compensate for that empty space.

How do Paul and Bassett start betting in "The Rocking-Horse Winner"?

The exact details aren't super clear; somehow or another Bassett discovered Paul's riding habit and his tendency to shout out names about the races.  Probably intrigued a bit, and part just humoring the kid in what might have been a passing fancy or fascination, Bassett lent him money for his first bet.  Paul tells his uncle, "he lent me my first five shillings, which I lost," and then he used ten shillings his uncle gave him to win his first bet.  From that point on, the bets most likely increased, and Paul was rather successful, because by the time his uncle figures out what is going on, Paul has over "three hundred pounds" on hand that Bassett keeps for him.  Paul and Bassett formed a partnership of sorts--this is beneficial to both of them, beause Bassett gets the names of winning horses, and Paul has an adult supervisor who can place bets for him and keep his money and holdings safe.  It works out well for both of them.

It's an interesting situation that eventually Paul's uncle wants to get in on.  Bassett is an honest man when it comes to dealing with Paul's money, and they end up making quite a bit of it.  I hope that helped a bit; good luck!

Describe Paul's first horse betting experience in "The Rocking-Horse Winner."

We learn about Paul's first experience betting after the fact. When Bassett, the gardener, implies that Paul has been betting on the races, Uncle Oscar takes Paul for a ride in the car. He asks him about the betting. Paul says that he put his first bet of five shillings on a horse and lost, then put down the ten shilling note Uncle Oscar gave him on the horse Singhalese. He won the race with that and decided Uncle Oscar was lucky. Paul has been betting ever since and has built up a substantial amount of winnings. He learns from riding his rocking horse furiously hard that Daffodil will be the winner of the next race.

Uncle Oscar decides to take him to the Lincoln races. We learn that:

The child had never been to a race meeting before, and his eyes were blue fire.

However, when Daffodil comes in first, the boy is "strangely calm" even though he has won the large sum of 1,200 pounds.

It is worth nothing that Uncle Oscar stumbles onto Paul's secret when his betting life is well under way and after Paul has won a great deal of money. Uncle Oscar is astonished at Paul's supernatural power. However, as we will find out, Paul does all this betting to bring his mother the luck, which to her mind is the same as money, that she feels she is missing in her life.