I'd agree that it's an individual thing about what's missing, but from my own perspective -- he forgot time. I've never been a very materialistic person, and in the past have had a hard time seeing "I love you" in a paycheck and the things money can buy. I know that's how a lot of people express their love, but everyone understands love in a different way.
For myself, it's the little things in life that TRULY show love. My husband spends time just talking to me, he loves to snuggle, and brings me my coffee in the morning fixed just the way I like it. He respects my opinions, and we can spend hours discussing some political, cultural or historic point. When he sees a frustration or road block in my life, he doesn't say a word -- he just looks for a way to fix it. As an example, I used to have an extremely messy desk that had too much stuff crammed into a small space and was a constant source of frustration for me. One day he just got up, said, "You're getting new shelves" and set about building them.
So...I guess from my perspective, he forgot the things that he can only learn through close observation and attention. Anyone can exchange money for items; it's not hard, and it doesn't necessarily take any thought. While many people see it as giving their beloved all that time they spent working for the money, others see it as being bought off to avoid truly meaningful interaction. If the relationship relies on materials, then it dies with the cash flow. I'll also add that I focus on what he forgot because that's the slant of the question, but no relationship succeeds or fails on the actions of just one person -- there are missing factors on both sides.