Answer:
The past ghost is a reminder of what once was. Scrooge, after being shown numerous events of past Christmas', shouts "No more!" and tries to put out the Ghost's light by covering the ghost; Scrooge wants to extinguish the Ghost.
While on their trips, you can see that the Ghost cares for Scrooge since the Ghost notices Scrooge trembling and crying from visiting past memories. Scrooge is reminded from these events that he would spend his past holidays alone, which is what made him sob out in the first place (well it depends on which version of Christmas Carol you're listening too).
The Ghost has good intentions, despite the Ghost's bright light bothering Scrooge.
(Again, the answer varies on which version you're reading but this one's pretty generalized. Sorry if it's confusing or it's not the best)