How come sometimes men last a long time in bed and some other times they don't last long at all?

Men's performance in bed can vary due to several factors, including physical health, mental state, and emotional intimacy. Stress, fatigue, anxiety, and alcohol consumption can negatively impact stamina, while a relaxed and comfortable environment can enhance performance. Additionally, arousal levels and the novelty of a sexual experience can also influence how long a man lasts. Overall, it's a combination of psychological and physiological factors at play.