Merge sort's best-case complexity is O(n log n) because: 1) Always divides array into halves, 2) Requires log n levels of recursion, 3) Performs n comparisons during merging at each level, 4) Independent of input array arrangement, 5) Consistent performance across all cases, 6) No early termination possibility, 7) Same number of operations regardless of initial order, 8) Predictable performance characteristic.