The United States Constitution is meant to protect the basic rights of all American citizens. The 10th amendment makes explicit the idea that the federal government is limited only to the powers granted in the Constitution, and so should have never allowed passage of the Defense of Marriage Act in 1996. The 14th amendment states "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."
Loving v. Virginia, 388 U.S. 1 (1967), was a landmark civil rights case in which the United States Supreme Court, in a unanimous decision, declared Virginia's anti-miscegenation statute, the "Racial Integrity Act of 1924", unconstitutional, thereby overturning Pace v. Alabama (1883) and ending all race-based legal restrictions on marriage in the United States. Same-sex marriage bans fall under the same exact illegal restrictions and deserve to be ruled as such.
These are the legal reasons.
What are your personal reasons?