He likely would have done the same had it been Amy in place of Andrea. The impression I get is that the bond he formed with the two sisters was such that his natural instinct was to act as a sort of father figure. With Amy dead, Andrea was all he had left of anything resembling a family. He didn't want to let go of that if he felt he didn't have to. Is this selfish? Well, yeah, sort of.... but then there's Andrea.... Andrea had just lost her sister and had discovered that the CDC was not going to be the savior of the human race like she had led herself to believe. As far as Andrea was concerned, there was nothing left to live for. Her solution, suicide, was maybe even more selfish than Dale's argument against it. Both had their own feelings in mind, but had Andrea not chosen to follow Dale out of the CDC, she would have affected anyone in the group who gave a damn about her, not just Dale. In convincing Andrea to press on, Dale really only affected Andrea's feelings toward him (as the "You son of a bitch!" look she gives him in the RV afterwards indicates). Jacqui being left behind is a bit strange, but less so if you accept that the character was created for the show for the single purpose of keeping Dr. Jenner from having to die alone.