For infants, drinking is about nutrition. Drinking formula or breastmilk is more important than eating until around the first birthday. After that, drinking should be driven by thirst. Unfortunately, it is often driven by a desire for a sweet treat. Some children also drink because they are bored or there is nothing better to do at the moment. Sometimes a toddler or preschooler is offered a sippy cup as a distraction or as a way to get quiet.
You can encourage good drinking habits with a few simple steps. First, encourage your child to sit down to drink. This means the child stops playing to drink. This also reduces mess-both spills and dripping cups on the floor. By 15 months, most children can easily walk into the kitchen and sit down to have a drink. Keeping a cup of water on a small stool works well. Pick up the cup off the stool and sit down to drink. When no longer thirsty, get up and put the cup back on the stool.
A one year old who sits down to drink will also learn to drink from an open cup more quickly. If you doubt that toddlers can do this, peek into a daycare room that requires toddlers to sit at the tables for snack. Each child drinks what they want before getting up to play. There are a few spills but 3-4 ounces is an appropriate serving size and does not make a huge puddle. A cup this size is also easy for a toddler to hold in one hand.
If you as an adult need to take a drink along on an outing then pack one for your child. Also consider a packing a drink if your child will be playing outdoors or engaged in another activity that causes him to sweat. Try to wait until your child should be thirsty or asks for a drink before pulling out the cup. If he throws it down, put the cup away because he is not really thirsty.