What’s the reason that every signal have a noise?

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Noise itself is a “signal” but instead of being deterministic and predictable (like x(t)=sin(ωt)x(t)=sin⁡(ωt)) it only can be defined and characterized through statistical parameters, such as average, variance, covariance and correlation. In signal processing, the noise is usually called a stochastic process. The most “famous” noise is white noise, which is characterized by a constant power spectrum (in the frequency domain) or, equivalently, by an autocovariance and autocorrelation equal to an impulse — δ(t)δ(t) in continuous noise, δ[n]δ[n] in discrete-time noise.

As others already said, in practice everything above absolute zero has intrinsic thermal noise; semiconductors have other types of noise (shot noise, popcorn noise, 1/f noise, …) and there is also the noise associated to electromagnetic interference from antennas, electrical appliances, etc…

So, noise is really pervasive; in practice, almost all engineering endeavors include some ways of dealing with (and be resilient and robust again) noise, or else the devices wouldn’t work properly.