By being there for them, showing them respect, doing things to help without being asked, and other such things consistently over time. Anyone can say "I love you" or engage in intimate relations with you, but by dedicating your time to someone, you're dedicating your life. This applies whether it's reciprocated or not and whether it's in a relationship or not. For instance, I have a friend who has loved me since we met, though he has only said so a handful of times and knows I don't return his feelings. The last time he said it was six years ago, yet his actions continue to show that his feelings have not changed. In my own relationship, my husband constantly shows he loves me simply by noticing things other people never would, and acting on them...he observes things about me that even I don't realize I'm expressing and uses that information to make me things I like, comfort me when I don't realize I've let on that I'm upset or frustrated, and so on.