The environmental reason that struck a cord for me is that plastic is made from fossil fuels (oil and natural gas), which take hundreds of thousands of years to form in the earth. They are non-renewable and, after a fossil fuel is burned, can never be used again.
Many single-use plastic products are used for less than a couple of hours before they are discarded. It doesn't make any sense to buy a single-use plastic bottle of water, for example, when we live in a country where we can get clean drinking water straight from the tap! It is such a waste of precious fossil fuels. My re-usable bottle is so much nicer to drink out of too.
The health reason that struck a cord for me is that since the late 70's we have been surrounded by plastic. It touches our food, our drinks, our medicines and everything else. Let's be honest. We are offered plastic shopping bags to carry food that is often packed in at least one layer of packaging or plastic. Although the use of some plastic elements (think BPAs) are being phased out of products that touch food we don't really understand the potential impacts of the other components yet.
Since plastic doesn't break down in the environment for millions of years it fragments and is eaten by wildlife, particularly in the ocean. Apart from being terrible pollution the plastic is then passed up the food chain to humans. How can we begin to understand the flow on health effects of that for not just humans, but all of the animals impacted within the food chain? Hormone mimicking effects (again, such as BPAs) are also realised through the food chain, particularly in the ocean.
The social impacts motivating me arose directly from Bag It. The movie talked about plastic recycling and how numbers 1 & 2 (in the recycling triangle) are the most readily recycled products (as opposed to numbers 3, 4, 5, 6, & 7). I know this is true because when I was working to increase plastic recycling at my work the only plastics that were collected (because they have a market) were numbers 1 & 2. Bag It explained that numbers 1 & 2 (which include PET plastic drink bottles) can easily be recycled into the lower grade of plastics (3-7). The lower grade of plastics, however, can't be recycled into a plastic of high enough quality to be economical. This means that plastic is generally only recycled once and then it becomes unusable waste.
When the higher quality plastics are collected and recycled they can be shipped to third world countries and processed in abominable working conditions. Workers sift through plastic waste and melt it down with no safety equipment. That is a pretty considerable social price for my plastic use.
On a more positive note I should mention what an amazing product plastic is. It can do so many things and it is so cheap to manufacture! In countries where there is no clean drinking water I'm sure many would suffer incredibly without it. It's like everything - let's try to use a great product responsibly so that we can continue to use it in a much nicer world for a much longer time.