Water wets glass because at any temperature, a fraction of the water molecules will escape the liquid form and evaporate into the gas form, water vapor. Air also has water vapor in it, depending various factors like temperature and altitude. This is what is known as humidity. Air can only contain a certain amount of water vapor. At a certain point, called a dew point, water droplets start to condense, and hence, form dew.
This is dependent on temperature. If the glass is colder, more dew is formed. This is because if the glass is colder, the air beside it also becomes cooler and cooler air "holds" less water. Then, the water droplets start condensing on the surface of the glass - both inside and outside, but at different rates.
Dew formation is also observed in a closed water bottle even in a hot day, and this illustrates some of the points provided above. In a hot day, air is warmer and can hold more water. Water molecules also have a higher tendency to escape the liquid form as they will have more energy due to the higher temperature. However, the cannot escape to the atmosphere due to a closed system. At some point, there will be too much water vapor that can be contained in the air inside the bottle, that it starts condensing on the inner wall of the bottle.
This process is not different from large scale evaporation in bigger bodies of water (water escaping the liquid phase), accumulation of the droplets in the air and then condensation (formation of clouds, until a certain point when it starts to rain) -- then the processes just cycle.
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