The main reason is because the company is expecting to finally be profitable, no matter how you crunch the numbers. In the shareholder letter, it says:
On the call, Musk said that he expects to have positive cash flow starting next month, using non-GAAP reckoning. The numbers sure look impressive when put into a chart:
We plan to be net cash flow positive and achieve non-GAAP profitability for the year, even after investing about $1.5 billion to add more production capacity, start cell production at the Gigafactory, and establish additional customer support infrastructure. Moderate GAAP profitability is expected in the fourth quarter.
The actual numbers from all of Tesla's books are not as pretty. The company is happy about projections but is still reporting a loss in the shareholder letter. It says that, "Our Q4  non-GAAP net loss was $114 million, or $0.87 per share, and our Q4 GAAP net loss was $320 million or a loss of $2.44 per share, both based on 131 million basic shares."
TSLA stock jumped up (11 percent as of this writing) in after hours trading. There were other bits of news as well. Tesla will start selling its vehicles in Mexico soon, and the company announced that the reveal for the Model 3 will take place March 31, for example, and we learned that the Tesla Autopilot software is learning how to drive for you, "at the rate of over a million real-world miles per day." Elon also said that he can see a time when Tesla will do some actual advertising, once it has a mass-market car to sell and as long as the ad is not one that people "regret seeing." Oh, and we can expect the first Model X reviews next month.
This post will be updated.