...does it mean there is somebody we don't love?
No.
When we say 'I love you!', does it mean that there is somebody we cold say to: 'I don't love you!'?
No. Why are you placing all these conditions upon the saying?
'I love you' - 'I love you not' - is it even up to us to say anything like it?
For me, it's more like, it isn't up to me to
feel it. Having been given it to feel, I feel it isn't mine to withhold.
Why exactly do we have to do anything?
The pressure I see, is born of culturally conditioned .....
Stop right there. No one is pressuring you, except whatever pressure you are placing on yourself. Perhaps that comes from your cultural conditioning, that pressure ... but I doubt that the reasonable response to it is to then attempt to invalidate another culture, or to invalidate some zany, crazy person who has the 'idiocy', in your view, to say, "I love you."
I'd guess, there are not so many people in the world who follow Dalai Lama's words: 'Love each other instead of being in need of each other'.
I would guess that you have no way of knowing this, and I would go further and say that some things are none of my business -- nor are they yours.
But... I am educated that you do not wish to be told "I love you." No more needs to be said. Point taken and application is forthwith.