Keith doesn't like using the term 'falling in love' when he thinks about his feelings for Shiro, because 'falling' implies an adrenaline rush and uncertainty and maybe even a mistake on your part, perhaps something that you would correct or do over, something to fix. Falling is the drop of a hoverbike against a cliff, failing a jump, crashing, hurt.

Instead, he thinks of it like gravity: once he was around Shiro's orbit, there was no way he wouldn't be attracted to it, even if it was something that started slowly, but there was no falling: there isn't, for him, a clear cut period of time when he can say 'here, Shiro did this and I knew I loved him' because it built over every gesture, every smile, each laugh they shared until it wasn't a fall at all, nor anything shocking, but the natural conclusion of something unavoidable: Keith walked into love with Shiro, and, as anything related to Shiro, even when knowing that his feelings are and always will be one sided, there is no hurt.

Perhaps it's selfish to think about it like that, perhaps it's wrong to think of your feelings as 'safe' but that is, really, just one of the many reasons in a very very long list of reasons why Keith doesn't tell Shiro how he feels.