"Kennedy chose Senate Majority Leader Lyndon B. Johnson, who had finished second on the presidential ballot, as his running mate. Johnson, a Protestant Texan, provided geographical and religious balance to a ticket led by a Catholic Northeasterner, but many liberals did not like the pick"
found this info from (https://en.wikipedia.org/wiki/1960_Democratic_Party_vice_presidential_candidate_selection)
The primary reason that John F. Kennedy chose Lyndon B. Johnson as his running mate is that Kennedy was seen as young and inexperienced while Johnson was a so called "Master of the Senate" that had served as a member of the House and Senate. Johnson also balanced out Kennedy's New England background with a dose of Texas.
In many ways, it is similar to Barack Obama choosing Joe Biden, although Obama treated Biden more as an equal while Kennedy isolated Johnson.
