We can only very indirectly and tentatively ascribe emotions/feelings to animals if we understand their brains functionally, and use functional imaging. This is difficult in practice. There is also a crucial difference in science between the subjective world (happy) and objective world (smiling).
In ethology, you can only observe behavior and its consequences. Stimuli can be appetitive and aversive, positive or negative. Insects, reptiles and mammals can similarly differentiate between sweet water (which they seek out) and bitter water (which they normally avoid). Those are easier behaviors to identify. Of course, in reality, an emotional state is made up of a complex multi-dimensional spectrum of multiple measures (satiety vs. hunger, anxiety vs. relaxation, happiness vs. unhappiness, fear vs. safety, sadness vs. unsadness, etc.) simultaneously. There is also a difference between the measurement and the internal reality. Additionally, there's the choice of measurement and the internal reality, too. One could measure happiness by width of your smile at a given time, but that would not be a very valid and reliable measurement. This is the general difficulty with measuring and detecting emotion - behavior is a more objective quantification.
If you were studying a new species of human, which could not talk or report to you using a language about how it 'felt', and had a brain significantly different from ours, we would also be unwise to ascribe specific emotions so callously. Not because the specimen would be incapable of feeling emotions, but because we wouldn't have the tools to check and verify internal emotional states, or probe whether the snake was conscious of an emotion regardless of its behavior.
However, we do believe that emotions require a big central nervous system to exist. Snakes certainly have large enough brains to experience fear and pleasure. It seems that most large vertebrates can, certainly mammals can such as mice. Whether reptiles or even insects have the 'cerebral capacity' for non-primitive (read: non-humanlike) is an open question.