Loyalty to masters. Some slaves felt a sense of loyalty to their masters and their families. They may have been treated well, or they may have felt a sense of obligation to the people who had raised them.
Fear of retribution. Some slaves feared that if they did not support the Confederate cause, they would be punished by their masters or by Confederate authorities.
Desire for freedom. Some slaves saw the war as an opportunity to achieve freedom. They may have joined the Union army in order to fight for their own emancipation, or they may have simply taken the opportunity to escape from slavery during the chaos of the war.
Belief in equality. Some slaves believed that the values of the Union, such as equality and democracy, were worth fighting for. They may have joined the Union army in order to help create a more just and egalitarian society.
The decision of whether or not to take sides during the war was a complex one for slaves. There were many factors to consider, and there was no easy answer.