Why is the dB plot of a signal usually non-positive?
In power spectra of signals, 0 dB is some agreed power level. It can for example represent the level of a full-scale (FS) sinusoid. In that case it would be rare to see values above 0 dBFS (sinusoid), as it would require peak cancellation as with $1.125\sin(x) + 0.125\sin(3x)$ to stay within a numerical amplitude range of $[-1, 1]$.  When a power spectrum is given in dB, the 0 dB reference should be specified. Otherwise one can't be sure what it means.

You saw the above 0 dB values not with a power spectrum of a signal but with a transfer function, or equivalently with a magnitude frequency response. In a magnitude frequency response, 0 dB means a magnitude or gain of 1, that there is neither attenuation or amplification. If you followed this standard convention and saw values above 0 dB, it means that the corresponding frequencies are amplified.