Consider positive functions as $f(n)=n$ and $g(n)=n^2$ for values of $n>=0$
Now, if C is a positive integer, of course $g(n)$ will be greater than $f(n)$ for all values of C
But, if C is a positive fraction, say $1/1000$ Then for some values of $n$ $g(n)$ will be smaller than $f(n)$
Hence, we have $C1 g(n) <= f(n) <= C2 g(n)$
Therefore, $f(n) = \Theta g(n)$