The purpose of the rainbow after the flood was a sign that the world will not be destroyed by a flood ever again (Bereishit 9:11):
" וַהֲקִמֹתִי אֶת-בְּרִיתִי אִתְּכֶם, וְלֹא-יִכָּרֵת כָּל-בָּשָׂר עוֹד מִמֵּי הַמַּבּוּל; וְלֹא-יִהְיֶה עוֹד מַבּוּל, לְשַׁחֵת הָאָרֶץ"
The rainbow serves us as a reminder that a sinful world was destroyed, just as we are 'sinners' now. Why then is the rainbow beautiful to look at? Shouldn't a 'reminder of sinful ways and subsequent destruction' be represented in a different way? e.g. perhaps the 'zocher habrit' blessing should be said on an actual flood, or torrential rain to remind us of the sin of the generation of the flood?
See here for brief outline of concepts in this question.