Answer by taking a quote from the following article:

Joseph Patrick Kennedy Sr. (September 6, 1888 - November 18, 1969) was an American businessman, investor, and politician known for his high-profile positions in United States politics. Kennedy was married to Rose Kennedy, and three of their nine children attained distinguished political positions: President John F. Kennedy (1917-1963), Attorney General and Senator Robert F. Kennedy (1925-1968), and longtime Senator Edward M. "Ted" Kennedy (1932-2009). He was a leading member of the Democratic Party and of the Irish Catholic community.

Kennedy wanted his eldest son, Joe Jr., to become president, but after Joe Jr.'s death in August 1944, he became determined to make his second son, John, president.  Kennedy was consigned to the political shadows after his remarks during World War II ("Democracy is finished"), and he remained an intensely controversial figure among U.S. citizens because of his suspect business credentials, his Roman Catholicism, his opposition to Roosevelt's foreign policy, and his support for Joseph McCarthy. As a result, his presence in John F. Kennedy's presidential campaign had to be downplayed.  However, Kennedy still drove the campaign behind the scenes. He played a central role in planning strategy, fundraising, and coalition and alliance building. Kennedy almost oversaw the entire operation, supervising spending, helping to select advertising agencies, phoning local and state party leaders, newsmen, and business leaders.  Kennedy connections and influence were turned directly into political capital for the senatorial and presidential campaigns of sons John, Robert and Ted. Historian Richard J. Whalen describes Kennedy's influence on John Kennedy's policy decisions in his biography of Joe. Joe was influential in creating the Kennedy Cabinet (Robert Kennedy as Attorney General although he had never argued or tried a case, for example). However, in 1961, Joe Kennedy suffered a stroke that placed even more limitations on his influence in his sons' political careers. Kennedy expanded the Kennedy Compound, which continues as a major center of family get-togethers.  When John Kennedy was asked about the level of involvement and influence that his father had held in his razor-thin presidential victory over Richard Nixon, he would joke that on the eve before the election his father had asked him the exact number of votes he would need to win: there was no way he was paying "for a landslide". Kennedy was one of four fathers (the other three being George Tryon Harding, Nathaniel Fillmore, and George Herbert Walker Bush) to live through the entire presidency of a son.

How did Kennedy encourage presidential ambitions?
Kennedy wanted his eldest son, Joe Jr., to become president, but after Joe Jr.'s death in August 1944, he became determined to make his second son, John, president.