It hasn't happened in the NBA since 2005. That year, the San Antonio Spurs (who won titles in 1999 and 2003) faced off with the Detroit Pistons (who won the previous year, 2004). Before that year, you'd have to go back to those epic Lakers-Celtics battles in 1984, 1985 and 1987 to see two title teams facing off.
It isn't just rare in the NBA. In baseball, the last World Series that had two championship teams battling for it all was back in 1963 when the Dodgers (who won back in 1959) and Yankees faced off (defending two-time champion). The last time the two defending champions faced off against each other in the World Series was back in 1943 when the Cardinals and Yankees met.
The last time the Super Bowl pitted two champions against each other was back in 1984. The Los Angeles Raiders (who won in Oakland in 1981) pummelled the defending champion Redskins. The last time the last two NFL champions faced off in the next title game was way back in ... 1943 again. The Bears (1941) and Redskins (1942) faced off in 1943.
In the Stanley Cup Finals, the last meeting of champions was 2001. The defending champion New Jersey Devils lost to the Colorado Avalanche (1996 champions). The last time the two defending champions faced off was back in 1976. The two-time defending champion Philadelphia Flyers faced off against the Montreal Canadians, who won in the year before that in 1973.
The point is that we rarely get to see two teams who know what it feels like to win a title actually play each other for the right to win another one. So even if you hate the Lakers and Celtics, you should watch. You don't know when you'll see two champions play again.