Corporate earnings growth remains supported by the outlook for global economic activity, which is improving.
The global outlook is OK, so that suggests that the earnings (growth) will still be there, and as long as they are, people take the view that the wherewithal for dividend growth and share buybacks will be there.
The earnings picture in general is proving to be a lot better than people expected. The outlook for earnings is still very positive so these markets are doing exactly as you'd expect. There's no indication yet they're looking tired.