There's a number of theories - some say that it didn't have berries until Frigga wept, and her tears became the berries so they signify a mother's love. Some say that Frigga decreed that the plant should no longer bring death, but love and life. Others say that that part of the myth isn't Norse at all, but Celtic, and it's associated with Christmas because of the Celtic solstice traditions that were brought into Christmas, and the kissing part was because it was associated with fertility in Greece and negotiating between enemies in Rome.

It's one of those parts of legend where there's no clear story, because one or all of them could be true.