"The difference between that moment in infinity war and this moment at the end of season 2 is that..... What I found so moving about Infinity war is that Loki refers to himself as an Odinson and so before his sacrifice it's really a private admission of love and respect for his brother" - Tom Hiddleston
The difference is that one sacrifice is personal, intimate and private whereas the other is for a greater good, for the multiverse, beyond Loki's personal needs and desires.