Besides the supremely unlikely "because they're just cool like that". Any pretenses towards the selfless or altruistic sharing of their vast cosmic wisdom just for the sake of welcoming us into the galactic neighborhood or whatever should be dropped at the door. What are some practical reasons aliens might want to uplift us to near their level of science and technology?
The ones I'm most stuck on are that they're doing it for either scientific or military purposes, and while the former is valid, "because we felt like it" doesn't usually make for a compelling story. So that leaves military purposes, like the Salarians uplifting the Krogans in Mass Effect. But why? Surely robots would be more efficient soldiers? Or is there some benefit to having human soldiers over swarms of mass-produced drones?