For one thing, both he and his idol Henry Clay were born there, although this is a superficial and not really accurate explanation.

In short, Kentucky was a slave state that he needed to keep in the Union. There were four such Border States (Missouri, Delaware, and Maryland being the others), all of which were absolutely crucial to Lincoln. The more states he had in the Union, the fewer states were in the Confederacy. It wasn't necessarily about manpower to him, although that is another reason, but about public opinion and eroding the idea of a slave state consensus.

Though Kentucky wasn't as immediately crucial as Maryland was (if Maryland had gone Confederate, Washington DC would have been surrounded and they would have had to move the seat of government, which would have been a huge blow to morale), it was a key battleground for the debate between slavery and abolition. Kentucky actually wanted to stay neutral when the war broke out, and it was only when Confederate soldiers marched through Kentucky (Lincoln had ordered his soldiers to go around if need be) that the state chose sides. But this kind of "well he started it" motivation wasn't really rock-solid, so Lincoln knew he had to appease Kentucky as well as the other states. (This is why, in the Emancipation Proclamation, Border States are excluded.)

I hope that answers your question :)

During** Darn auto correct