Reversal of meanings

I've noticed an odd pattern on Modern English. Words that are adapted
to mean "bad" (such as "sick" and "sh*t") have switched meaning to
mean "good" (i.e. "That is SICK" and "give me some of that sh*t").
Contrariwise, the word "awful" switched meaning (albeit earlier) from
"full of awe" and "wonderful" to "terrible."

Why do these shifts occur?

And aside from the religiously forced shift of the Indic gods to
demons in Aryan cultures, is such a shift actually odd, or is it a
factor that sometimes, even often, needs to be taken into account in
tracing words to proto-languages?

I know this isn't specifically Indo-European, but it certainly can apply.

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Also, which is Proto-IE more closely related to - Semitic or Uralic?

If the use of *m- and *t- for first and second person is not a
borrowing (and it would have to be of a VERY intimate nature to have
such influence), then wouldn't that make Uralic closest to Proto-IE?