It looks like you're looking for a conjunction or transition word to complete the sentence about spring. Based on your provided information, the correct choice to connect the ideas would be "because."
So it could be framed like this:
"Spring is the best season of the year. One reason is that the temperature is not too hot or too cold. Spring is the best time of year to start a new routine because it is much easier to get up and go for a run in the morning if the weather is pleasant."
If you need further assistance or clarification, feel free to ask!