Five precepts are not about forcing oneself to do something unnatural that reduces one's enjoyment of life. It's not like that at all! :)
Instead, in the beginning we tend to be a little confused about ways to enjoy life, and while thinking we're pursuing enjoyment, we sometimes hurt ourselves. It's like a kid eating playdough thinking he enjoys it but in fact only getting sick in the stomach.
What precepts are really about is our mental and emotional stability. They are the opposite of getting carried away and eating playdough.
- To abstain from killing.
What this really about is anger management. Every time we let ourselves to get carried away by anger we end up doing/saying stupid things and then regretting it later. So being able to control anger and (verbal or actual) violence based on anger is beneficial to us and to others.
- To abstain from taking what is not given.
This is about desire management. Every time we get an urge or an obsession to posses something - that's not us anymore. We become slaves of an idea or an image we have caught somewhere. It's like a mental virus that makes us sick with disease of wanting something. We are not free anymore. So being able to not get obsessed with desires is a matter of mental/emotional health, that leads to freedom to chose what we do based on rationally expected result, not on an irrational urge.
- To avoid sexual misconduct.
This is similar to the above. Desire to enjoy pleasant sensations gets us into a condition of tunnel vision. We no longer have clear idea of what we do, what are its sideeffects, what results we may expect for ourselves, and how we may be hurting others. Instead, our actions should be based on clear and rational understanding of the expected results, not on seeking pleasure. Sexual pleasure is one case, but pleasure of any kind is a false misleading promise. We may enjoy eating five pounds of icecream, and then get sick with a cold. Instead, if we eat strategically, with health in mind, we end up being healthy. Similarly, if we engage in sex with the goal of communicating our feelings for the other person, and with the goal of procreation - we end up creating the positive effect, because we are not blinded by desire, we see and know what we are doing every step of the way.
- To abstain from false speech
False speech and negative speech are almost always motivated by fear and hatred. It either is done in order to protect oneself, or to put the blame on the other or both. It seems that by engaging in false speech we're benefiting ourselves. But in fact we hurt ourselves a lot more, because by doing that we're increasing division and isolation, we build an invisible glass wall between ourselves and others. The more we do it, the lonelier we become.
- To refrain from intoxicating drinks and drugs which lead to carelessness
This one is obviously hurting oneself by putting oneself in the position of reduced mental capacity, confused understanding of one's environment, and decreased control over one's actions. Driving while drunk is an obvious example, but the point here is not even literal intoxication, it is subjecting oneself to any situation when one is confused and out of control to the point when one is hurting oneself. This could be indulging in video games, or watching too many TV series, or hanging with bad company - immersing oneself into any strong influence that leads to loss of critical capacity and awareness is a way to make oneself helpless and eventually hurt oneself.
So as you can see the five precepts are not about being harsh on ourselves - it's the opposite. Five precepts are about taking care of ourselves by being balanced, rational, and aware of one's state, one's actions, and their results.