Before I answer this question, I must preface that any intimate engagement must be consensual for both individuals, with the view of entering a relationship that will be a long-lasting commitment.
Random encounters are typically unfulfilling, usually leaving one or both parties feeling empty. I will make the assumption that you are referring to a healthy, monogamous relationship.
According to the Mayo Clinic, the DHEA hormone is released during arousal.
This hormone gives your immune system a bit of a boost, helping your body fight bacteria, viruses and other germs.
The hormone is also said to act as an antidepressant, and may even increase apprehension and learning abilities.
An article in Psychology Today mentions the endorphins released during intimacy actually help you having fewer worries and can significantly lower blood pressure.
Whereas not having sex with your partner may make you notice that your stress level builds up, highlighting intimacy and physical contact are things humans need to survive.
A 2010 report, Sexual Experience Promotes Adult Neurogenesis in the Hippocampus (a brain region linked with memory), by Leuner, Glasper and Gould, revealed that having sex changes your brain chemistry in all kinds of ways, increasing your cognitive capacity.
Even just thinking about a past intimate encounter can enhance analytic thinking skills. In one study, sexually active rodents had more neurons in their hippocampus than virgin rats.
However, improvements in brainpower were lost after sexual activity was stopped.
An article in Archives of Sexual Behaviour by researchers Maunder, Schoemaker and Pruessner (2016) highlighted that blood flows during intimacy, carrying a surge of nutrients and oxygen to the brain cells and activating every part of the brain.
By contrast, popular brain activities such as doing Sudoku, crosswords or playing memory games have marginal efficacy and each engage only a handful of brain regions at best.
Additionally, the research supports a positive association between intimacy and recall for words in women.
Therapist Michele Weiner-Davis, in her book The Sex-Starved Marriage, states that intimate relationships boost self-esteem, make a person feel younger, lower the level of cortisol (a hormone that can trigger fatigue), lower feelings of insecurity, keep spouses connected emotionally, help to give people a more positive attitude, make a person more calm and less irritable, reduce depression and relieve stress.
Having an intimate relationship with your partner can result in many interacting psychological factors and can vary significantly between people.
Nevertheless, it can account for mental health benefits from reducing mild headaches, improving sleeping difficulties and easing muscle tension to improving one’s immune system and fending off depression.
Evidence shows being close to your partner can trigger the release of all kinds of chemicals in the brain including dopamine (which plays a major role in reward-motivated behaviour, focuses attention and generally increases motivation), endorphins (our body’s natural pain and stress fighters) and oxytocin (which can trigger feelings of compassion).
If you are having difficulty in having an intimate relationship, talk to your medical practitioner or seek out a local psychologist who specialises in relationship counselling.