I got the impression that when Madara mixed Ashura and Indra's chakra they connected and hence the sage's chakra was brought forth in the process of awakening the rinnegan.
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Sasuke is undergoing something very similar so its likely that within Sasuke Ashura and Indra's chakra's are also being combined since he is getting hash-roids pumped into him. This brought forth the sages chakra in Sasuke hence why he is now talking to the sage in his mind.
Naruto's case was different. The sage felt that Ashura's chakra was attached to Naruto's spirit but Naruto's personality itself drew the sage to Naruto. Naruto himself summoned the sage because he has a unique power to draw people in and connect them.