Most heroes that are popular and have been around a long time build up a strong network of supporting characters and/or sidekicks. It gives them something to fight for, people to protect, goals to work towards, different personalities to play off.
Batman - Alfred, Dick, Tim, Jason, Damien, Stephanie, Cassie, Helena, Catwoman, Gordon, Bullock, Leslie Thompkins, Ace, Renee Montoya, Barbara, Lucius Fox, etc
Superman - Supergirl, Superboy, Jimmy Olsen, Lois Lane, Perry White, the Kents, etc
Flash (depending on your favorite) - Barry Allen, Wally West, Bart Allen, Jay Garrick, Joan Garrick, Iris, Patty, Singh, Frye, Pied Piper, Solovar, Max Mercury, Johnny Quick, Tina McGee, Jesse Quick, the two sets of Twins, Linda Park, Henry Allen, etc
Green Arrow - Roy Harper, Mia Dearden, Connor Hawke, Black Canary, Hal Jordan
Not to mention friendships (Barry-Hal-Ollie), rivalries (Bats and Supes), team affiliations (JSA, JL, Titans, Young Justice, Birds of Prey, etc)