Major League Baseball rivalries are fueled by geographic proximity, frequent competition, and cultural ties, with their intensity shaped by decades of scheduling changes. Starting with expansion in 1961 (and the 1969 divisional split), MLB repeatedly altered how often teams faced divisional foes, culminating in the 2001 "unbalanced schedule" that increased intra-division matchups. The Houston Astros' move to the American League West in 2013 prompted year-round interleague play, and by 2023, intradivisional games were reduced to 13 per season while interleague games were standardized. A new MLB Rivalry Weekend debuts in May 2025, featuring mostly interleague matchups between designated rivals. The most iconic example remains the Yankees–Red Sox rivalry, a 120-year-old feud considered one of the fiercest in American sports, highlighting how historical competition defines the league's drama.