First, a note: "harsh" and "soft" aren't standard linguistic terms, nor is "an aggressive spirit". So I'm going to ignore those parts and focus only on the specific sound change you brought up.
Front rounded vowels and back unrounded vowels are relatively uncommon, cross-linguistically: there's a trend for front vowels to be unrounded, and back vowels to be rounded, for articulatory reasons. You can see a map of where front rounded vowels appear here; the white dots (no front rounded vowels) are by far the most common.
However, there are a few different patterns which can produce them. For example, a chain shift can push a high rounded back vowel forward, with the other back vowels rising to compensate. This is what happened in both French and Ancient Greek. Alternately, vowel harmony can spread the "rounded" or "front" feature from one vowel to another, creating new combinations. This is what happened in German, Old English, and Finnish.
Now, according to every reconstruction I've ever seen, Proto-Indo-European had no front rounded vowels. Some languages developed them, as far as we can tell, entirely by random chance, like Ancient Greek. However, looking at the map, front rounded vowels are quite strongly clustered in certain parts of Europe, and much rarer outside that area. This is called an "areal feature", and probably started in a single language or family and spread to others in the area via contact.
So it's quite possible that e.g. an ancestor of the modern Uralic languages developed front rounded vowels via harmony, and they spread from there into local branches of Germanic, Romance, and other families. As Maddieson puts it:
In view of the different historical scenarios which produced the front rounded vowels in various languages, it is quite striking that their occurrence is so relatively concentrated in a particular geographical area. It seems likely that the hearing of sounds of this sort in some languages of the area may have given further support to phonetically natural processes in other languages, with the end result being the addition of front rounded vowels to the inventory of more of the languages.