The beauty of the Internet is you can pull up anything very quickly. Here are some states declaration of secession. In the first sentence or two, the states make it incredibly clear why they seceded from the union
Georgia:
The people of Georgia having dissolved their political connection with the Government of the United States of America, present to their confederates and the world the causes which have led to the separation. For the last ten years we have had numerous and serious causes of complaint against our non-slave-holding confederate States with reference to the subject of African slavery.
Mississippi:
In the momentous step which our State has taken of dissolving its connection with the government of which we so long formed a part, it is but just that we should declare the prominent reasons which have induced our course.
Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world.
South Carolina:
The people of the State of South Carolina, in Convention assembled, on the 26th day of April, A.D., 1852, declared that the frequent violations of the Constitution of the United States, by the Federal Government, and its encroachments upon the reserved rights of the States, fully justified this State in then withdrawing from the Federal Union; but in deference to the opinions and wishes of the other slaveholding States, she forbore at that time to exercise this right.
Texas:
The government of the United States, by certain joint resolutions, bearing date the 1st day of March, in the year A.D. 1845, proposed to the Republic of Texas, then a free, sovereign and independent nation, the annexation of the latter to the former, as one of the co-equal states thereof.
The people of Texas, by deputies in convention assembled, on the fourth day of July of the same year, assented to and accepted said proposals and formed a constitution for the proposed State, upon which on the 29th day of December in the same year, said State was formally admitted into the Confederated Union.
Texas abandoned her separate national existence and consented to become one of the Confederated Union to promote her welfare, insure domestic tranquility and secure more substantially the blessings of peace and liberty to her people. She was received into the confederacy with her own constitution, under the guarantee of the federal constitution and the compact of annexation, that she should enjoy these blessings. She was received as a commonwealth holding, maintaining and protecting the institution known as negro slavery-- the servitude of the African to the white race within her limits-- a relation that had existed from the first settlement of her wilderness by the white race, and which her people intended should exist in all future time. Her institutions and geographical position established the strongest ties between her and other slave-holding States of the confederacy. Those ties have been strengthened by association. But what has been the course of the government of the United States, and of the people and authorities of the non-slave-holding States, since our connection with them?