Your house/apartment consumes an overhead of energy no matter how many people lives in it. Share your space and you divide up that load on the environment amongst you and your housemates!
It takes the same amount of energy to heat/cool your house no matter how many people are in it. A certain amount of electricity is used (refrigerator, furnace fan, internet connection, parasitic loads, etc.) no matter how many people are there.
By taking on one housemate you'll half your heating/cooling emissions footprint. Take on two and you'll cut it to one third!
Plus you're sharing the material/resource loads of the house and all your furniture and appliances.
If you don't believe this try taking a carbon calculator test. It'll ask you how many people live in your house. You'll find it makes a huge difference.
And as a bonus you'll save on living costs!