Why won't Jesse allow Winnie to drink from the spring?

Jesse won't allow Winnie to drink from the spring because he understands the serious consequences of immortality that come with it. He wants to protect her innocence and allow her to experience life fully, including its natural cycle of aging and death. Jesse believes that living forever is not a blessing but a burden, and he hopes to spare Winnie from that fate. Ultimately, he cares for her and wants her to make an informed choice about her life and future.