Mazel means a flow from above… tov means good or: a good flow from above. Mazel tov means may you be aligned with your soul above… May you appear below the way you are seen in your pristine essence above…
So mazel tov for the marriage really means may your marriage reflect the way a marriage should be from G-d’s perspective. Mazel tov upon your birthday means may the coming year below reflect the way it is seen in its full potential above!
Pretty deep blessing!!
Menachem Bluming and Chabad.org