Your cell phone is just a radio transceiver that operates behind a sophisticated user interface. Anything that can interfere with radio frequency signals can cause cell phone reception problems.
Cellular communication works on a line-of-sight basis. If you can see the tower, then you have a pretty good chance to get a reliable cell signal.
That said, there are two basic reasons why cell phones drop connections:
1) You are too far away from the cell tower.
The signal cannot reliably cross the distance to your phone. Or much more likely, the relatively weak transmitter that is your cell phone cannot reliably send its signal across the distance to the tower.
2) There is something (or multiple things) between you and the cell tower that's interfering with the signal.
If you're inside your home or another building when you drop a connection, it's most likely the materials used in construction that are blocking the signal. Brick, stone, concrete, steel, radiant barriers, even the metal oxide-coated low emittance glass in your windows don't allow cell signals to pass through.
Same story when in your vehicle. You're inside a shell made of metal and coated safety glass. No wonder the cell signal has trouble penetrating.
Even when you're outdoors things like trees (yes, trees!), bad weather, hills and other terrain, dust in the air, and any nearby structure can block signals.
To learn more about why you lose connections, check out our video below, or read this blog post Why Do I Have Such Poor Cell Phone Reception?