Eren giving Levi a rose for every week they’re together. So Levi ends up buying a big vase, because Eren insists that he saves them all. So in the future they kinda have a fall out, and all Levi has is his roses.
After a while Eren tries to reconcile, and Levi lets him. It takes some time but they work things out again. And one day Eren brings Levi one hundred roses, symbolism of a love that won’t crack for hundreds of lifetimes and I can’t.