First point: Chinese Buddhism (Chan Buddhism, which extended into Korea as Seon and into Japan as Zen) is markedly influenced by Daoist philosophy. This is typical of Buddhism in general, which tends to absorb cultural elements as it moves into new regions, instead of replacing or suppressing them as other religions do. In fact, one of the major differences between Hinayana and Mahayana traditions is a shift in orientation: losing some of the traditional emphasis on reincarnation and the karmic cycle and gaining a more humanistic worldview traceable to traditional Chinese worldview.
Second point: it's a mistake to consider Buddhism as life-denying in any of its forms. Buddhism considers cravings, attachments, obsessions, fixations, etc. to be life-denying elements — a false view of the world that merely increases human misery - and holds out a life-affirming possibility of liberation from such misery. I mean, sure, if someone is completely infatuated with the idea of obtaining a new car (or a new girl/boyfriend, or a high-status job), it may seem obvious to them that achieving such would be life-affirming. But Buddhists notice the struggles, the trials, the failures, and the unwanted mental states that arise in the single-minded pursuit of such external goals. The realization that these external goals are effectively empty victories (or empty failures) is where Buddhists find the affirmation of life.
Third point: All moral philosophy bends towards the same pattern, in the same sense that all green plants bend towards the light: see the perennial philosophy. It's not easy to put this pattern into words, and there is a constant cycle in which the pattern is expressed, misunderstood, corrupted, and then expressed again in a new form. So even if Daoist principles had not been absorbed by Chan Buddhism, there would still (naturally) be common themes and common insights, separated mainly by language, convention, and symbology. It's unavoidable.