A very stressful or intense situation, or performance anxiety, could definitely make a man lose his erection. The pressure to perform could be psychologically too intense to keep a strong one. Another, lesser talked about erection killer is difficulty penetrating a partner during sex. It’s related to performance anxiety and stress, but also fatigue. If sex becomes tiring, fatigue will bring a quick end to an otherwise great night.
Don't forget mental health, either! "If you are experiencing stress at work, in your relationships or at home, open up communication about these issues," notes Axe. "Try natural stress busters like spending time outdoors, taking some vacation time for yourself or seeing a therapist. You also need to make sure that you are getting enough rest every night — seven to nine hours of sleep per night."
In an article on Men’s Health about things that wreck your erection, some examples of threats to an erection are sugar, lack of sleep, sleeping near a newborn baby, and lack of vitamin D. The common theme in the article is that reduced testosterone impacts your erection. Not all scientists agree with that. But there are definitely links between testosterone and erectile problems. 

what is the main cause of erectile dysfunction young men

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