There are many events occurring on the I2C bus but without a logic analyzer or an oscilloscope you won't be able to "see" them.
START - the device (usually the MASTER) issuing the START condition first pulls the SDA line low. And next pulls the SCL line low.
STOP - the bus MASTER first releases the SCL and then the SDA line
ACKNOWLEDGE - the addressed SLAVE device or the MASTER pulls the SDA line low, depending on the current operation
Fig 3. START and STOP events
The START condition acts as a signal to all connected devices that something is about to be transmitted on the I2C bus. The STOP condition tells the connected devices that the message has been completed.
The ACKNOWLEDGE is given by an I2C device (MASTER or SLAVE) after every byte is transmitted.
Fig 4. A 1-byte packet on the I2C bus.
The MASTER should give an ACKNOWLEDGE after receiving a byte from the SLAVE in a read operation. The SLAVE should give an ACKNOWLEDGE after receiving a byte from the MASTER in a write operation. The SLAVE should give an ACKNOWLEDGE if it is being addressed. If there is no ACKNOWLEDGE the SLAVE is not on the bus.
Fig 5. SLAVE being addressed on the I2C bus. A6...A0 is the 7-bit device address.
These events are necessary for the correct operation of the I2C bus. MCUs that have hardware support for I2C perform all this behind the scenes. If you have an MCU that does not support the I2C bus but have 2 extra I/O pins, then all is not lost! You can bit-bang the bus.