In four hours, the oxygen will run out; they will likely be saved (Pidge is a genius, and the rest of their team is so stubbornly brave) but, just in case they aren't....

"Keith, I never got around to telling you - the time never seemed right - but just in case we're out of time, I want you to know... I love you too." Shiro says, taking up valuable oxygen in order to utter something even more important.

Kissing takes up even more oxygen that they potentially don't have, but ... even if this wasn't potentially the end of the line, Shiro would not be able to deny Keith.