I certainly hope I'm not reborn as an animal in a future life. However, Buddhism does teach that one can be reborn in the animal realm. The animal realm is considered one of the unfortunate births. So in order to be sure that you are reborn human there is a requirement, written in the scriptures, that you have to keep the Five Precepts.
The first precept is to undertake the training to refrain from killing living beings. The second precept is to undertake the training to refrain from taking what is not given. The third precept is to undertake the training to refrain from sexual misconduct. The fourth precept is to undertake the training to refrain from wrong speech. And the fifth precept is to undertake the training to refrain from taking intoxicants which confuse the mind.
According to the scriptures, in order to be reborn human we have to try to keep these Five Precepts. And you can actually observe and understand this from your own experience in your normal life, you don't have to blindly believe this.
When a person takes intoxicants that confuses their mind, can they think clearly as a normal wise human being? They can't think clearly like a wise human being, it is like they automatically drop down to a lower level of being. And this will often show in their actions, they will commit all these other immoral acts. They will lower their ethical standards or they will not keep to them. They can't think clearly, and because of that, they may take what is not given to them, thus stealing, or lying, or killing living beings due to their unawareness, or committing sexual misconduct. You can see for yourself that people whose minds are confused by intoxicants already drop to a lower level in this life than that of a wise human being.
So if you want to be a wise human being, it helps to keep the Five Precepts.