Why is photosynthesis an endothermic reaction?

It's sucking up energy from its surroundings. Because that's probably not nearly enough information to answer your question, keep reading...

An endothermic process is one that needs to have energy added in order to take place. A simple example of an endothermic process is the melting of ice, because ice doesn't melt unless you put energy into it.

In the case of photosynthesis, nothing much happens until light hits the chlorophyll in the leaves, causing electrons to move around and a whole bunch of other complicated stuff to take place. The specific stuff isn't the important part - what is important is that without the addition of energy in the form of light, nothing will happen.

OK.