"I'm never going to leave you again, never,"  Marcus promised her, "I will do anything and everything I can to always be with you, even if it means flying a Viper," His attempt to lighten the mood felt childish but he needed it to go on.  "I am never leaving you."  He was struggling to find words to convey how much he meant this, wanting her to understand that he just couldn't function without her but not knowing how to tell her.  That living without her would be like living without the ability to breath.  He would be suffocated in the emptiness at his side that was supposed to be her.  And making this promise, to never leave her and that he loved her, terrified him.  It was a surrender.  It was him giving up the idea of being independent and strong on his own, by telling her this he was exposing a weakness that would forever be raw, his need for her.  
to stave off these thoughts he whispered those three words to her again before his lip's crashed over her's, tasting her tears but needing to show her he meant this.