Why do I have to open Serial Monitor to display what is in loop function?
Hard to tell from your fritzing which actual ESP8266 pin you are using, but you require a pull up resistor on the data line of the DHT22.

My guess is that you are using one of the serial UART pins for the data connection, and opening the serial monitor activates the output of the onboard UART chip which then acts as your missing pullup resistor.