The Gemara in Pesachim (49a) describes marriage as the combination of Invey Hagefen, twogrape vines. Why grapes? Also, the Gemara in Berachos (50b) states that the custom was to pour wine back and forth before the Chosson and Kallah. Why wine?
The Ben Ish Chai explains that normally when food and liquids age, they get spoiled and ruined. Wine, however, has the special quality that it gets better with time! People will pay a fortune for vintage aged wine! So too, we bless the newlyweds that their marriage should have the quality of wine, it should always get better with time. The relationship and love should continuously deepen and become stronger!
Reb Shlomo Zalman Auerbach offers another thought. If you eat an apple, the berachah pronounced is, “borey pri ha’aitz”. If you liquefy it into apple-juice, the berachah is downgraded to sha’hakol. So too, with all fruits, when they are blended, their berachah is not as great as when they were a fruit. This is not the case by grapes! When you blend grapes, they become more chashuv, significant, and their berachah is thus upgraded to “borey pri hagafen”! We give a berachah to the Chasson and Kalla that they too should be like grapes and blend together and be elevated and upgraded through the experience!
I add that this may very well be hinted as well by all of the wine that we pour down the Chasson and Kallah’s throats. From the time that they stand under the Chupah and throughout all of the Shevah berachos, they drink much wine! I believe that we are blessing them that they should have a beautiful wine-like marriage. L’Chaim!