Ever since their debut, Sonic the Hedgehog and Tails have always had a special relationship. In the Sega Genesis games, Tails was Sonic's invincible buddy who copied everything Sonic did, sometimes bashing a few robots and even Robotnik in the process. When Sega gave Tails the gift of flight in Sonic 3, he could carry Sonic to new heights so long as he didn't get tired first. Though the two are practically brothers, Sonic broke the first rule of brotherhood.
All of the above occurred in Archie Comics' discontinued Sonic the Hedgehog comic book series. When Tails' parents went missing, Tails lived at Sonic's house and the two slept in the same room. Sonic also brought his little buddy on missions with him. But in issue #155, Sonic betrayed Tails in one of the worst ways imaginable. He stole the girl of Tails' dreams, Fiona the Fox, whom he'd had a crush on since issue #28.
It's not as though Sonic wasn't aware of Tails' feelings. He once spoke to Fiona about how Tails felt to make sure she wouldn't lead him on and let him down gently. Despite that, Sonic still began sneaking out with Fiona behind Tails' back. Sure, Sonic had recently experienced his own heartbreak. He and Princess Sally had just split up, and when he first sought the affections of Mina the Mongoose who'd professed her love for him earlier on, she turned him down because she was now in her own relationship. But still. There were plenty of other girls who were attracted to Sonic. So why Fiona?
Regardless, Tails eventually caught Sonic and Fiona sneaking off together into the woods. The confrontation was heated. Tails later went so far as telling Sonic that he hated him before running off, heartbroken. But the betrayals didn't end there. Sonic continued seeing Fiona regardless of how much it hurt his buddy and would even flaunt their relationship in front of him. What's worse, all of Sonic's thoughtless interactions would normally result in Tails running off, crying. Each time, Sonic would usually have the audacity of asking what was wrong with Tails.
Even though Tails was only 11 at the time and both Sonic and Fiona were 16, Sonic should have been more aware of his little buddy's feelings. In fact, Sonic later told Tails he hooked up with Fiona because he'd hoped that Tails would eventually move on. This proves he did know how Tails felt. While Sonic and Tails would eventually make up later on in issue #179 with Sonic admitting he had been a selfish jerk, it's important to note that Sonic and Fiona had just broken up. Would Sonic the Hedgehog have called off the relationship for Tails at this point? It feels unlikely.