Hey Haewon! 

Thanks for the kind message! Happy New Years to you, too! =))

Yeah, this sentence is rather tricky since it uses an old(er) style of English that isn't commonly used today. You did a great job inferring something positive. Yes, Dickens considers educated Americans very highly and positively.

"entertain" more closely means

- to hold something in your mind

or

- to be willing to consider/accept something

"(I) never can make again so many friends for whom I seem to entertain the regard of half a life"

for whom I seem to entertain the regard of half a life --> Because of these friends, I am willing to consider a deeper appreciation for the life I have lived up to this point.

In a nutshell, Darwin is talking about the impact that his American friends had on him, that they helped him to have more appreciation (regard) for life. "Half a life" refers to Darwin being a middle aged man (maybe in his 40s) and having lived half a life. 

Hope this helps! You did the right thing by using the context to help you out; you just probably got a little confused by the sentence. It happens sometimes. =)

You got this!