My wife and I both came from families where we very rarely heard those words, not to mention physical affection wasn't the norm for either of us in our families. I know for me this carried well into my adult life. I was very careful to ever say the words even when I felt them. This hampered my emotional growth quite a bit, in my opinion.
It comforts me to know that the last words I said to my Daddy before he died were, "I love you, Daddy." His last words to me were, "I love you, too, son." I don't remember us ever saying those words before then, even though they may have been said. We knew how we felt, but we just couldn't bring ourselves to say the words. Why?
It was only after my Daddy's death when I finally found the courage to say those words when I felt them about somebody, especially family members. After a while they became much easier to say.
Fast forward to a couple of LTRs where I said those words, and the reciprocated words were hollow, meaning I ended up with a broken heart. Needless to say, I learned a little about emotional maturity, and to use those words only AFTER I knew how my SO felt, whom, it turns out became my wife. I won't say the words without meaning them, so if I say them, I mean them.
After losing my Daddy, my Moma (two years later), and two of my brothers, I make it a point to tell my family members that are left that I love them. I also very freely use these words to my wife and two DDs, and they are reciprocated always. I am happy to say that my family is extremely close and affectionate, something my wife and I didn't grow up with. We leave NO doubt now.
It doesn't come from a point of "weakness", either. Being "weak" is not in my vocabulary. It comes from a point of courage, in my opinion. Anybody who knows me knows that there is nothing "weak" about me, and most admire my courage to say, "I love you", despite whomever may be listening. At least that's been my experience.
It reminds me of the Garth Brooks song, "If Tomorrow Never Comes". Why not have the courage to say those words to the ones you love? Will they ever truly know how you felt about them if you don't? Actions speak louder than words, but if the actions are backed up by the affirmation of the words, there wouldn't be any room for doubt.