A love interest doesn't have to mean literal love. It's the inclusion of another character for the primary character to fall for. Almost always someone of the opposite sex who the main character is attracted to and feels a need to protect/long for. There's this mentality in Hollywood (and everywhere else for that matter) that almost every hero needs someone to protect outside of the masses, the everyday, random people who need help.
Superman can't just feel the need to protect the world, he has to have someone in particular to protect. Spider-Man can't just be the protector of New York, he has to save his girlfriend too.
In some stories like Superman/Clark Kent and Lois or Spider-Man/Peter Parker and Mary Jane or Gwen Stacy it's pretty much ingrained in the history of the character permanantly. There's a long, steady history of these two characters together that has come to define who they are. That's fine. Without their love interest it would feel incomplete. With the Fantastic Four and Reed/Sue, they're already together so it doesn't count.
Batman/Bruce Wayne and whoever the love interest of the movie is (there have been seven or eight movies and every time it's a new woman excluding Catwoman in two seperate franchises and Rachel who barely counts in TDK) gets shoe-horned in too often to give the character extra incentive/plot device to do something.
Batman doesn't need a love interest to do his heroics. Neither would someone like Frank Castle as the Punisher or even Tony Stark although his was well done and justifiable.
In Thor, Jane is meant to serve as his human connection but it wasn't necessary to be a love interest, it's just their chosen route to go. It's quick, easy and cliche'd as hell. Bruce Banner/Hulk and Betsy wasn't necessary but again, it's his link to his humanity.
Daredevil with Elektra was just pointless.
In fact, other than Blade, who still sort had one in Blade II (I'm split on whether Karen from the first one is an actual interest) is the only one I can think of offhand besides The Punisher (which inverts it to love interest/family lost) to be a super hero who doesn't need a love interest to function as a hero.
This is a tried and true story telling method but it's also tired and I would like to see something that does it differently for a change.
) that you cant possibly explain it all unless the film was like two days long.
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