Why did President Lincoln abolish slavery?

This is an interesting question, and it would be encouraging if the answer were a straightforward one in which Lincoln freed the slaves because he thought slavery was hateful and evil.
In fact, he did think that (he had some racist ideas, but he thought slavery was a horror). However, his first goal as president of the United States was to preserve the Union.
When the war began, Lincoln was willing to allow any state that decided to rejoin the Union to continue the institution of slavery. He wanted to keep the country intact above all else, but by 1863, when it was clear the South had dug in for a long fight and would not capitulate until they were militarily defeated, Lincoln changed his tactic. He freed the slaves and hoped they would join the union army to fight on the side of the North. He thought the loss of black manpower in the South and the added troops on the Northern side would help hasten the end of the war.