After United Parcel Service (NYSE:UPS) missed earnings estimates for two consecutive fourth quarters, investors would be entitled to feel a bit concerned about the company's direction. It's fair to say UPS' execution has not been what investors have hoped for, but that doesn't mean the stock isn't worth buying. On the contrary, there are many reasons to feel positive about the stock, and the recent dip could prove an excellent entry point. Let's look at the three key bullish arguments behind buying the stock.

Getting peak demand right
Some readers will already be familiar with the reasons behind the UPS fourth-quarter earnings miss and the five things management wants you to know about its recent earnings. Simply put, the shipping specialist has struggled to manage the cost of dealing with increasingly large surges in peak demand over the holiday season. In 2013, it had undercapacity and was forced to spend extra to ensure deliveries; in 2014, management overcompensated and built out too much capacity.

First, investors shouldn't assume UPS will always be unable to meet its expectations during the holiday season. It's easy to be critical of UPS management for missing estimates, but consider how e-commerce has increased the complexity of the trading environment during the holiday season.

As CEO David Abney said on the recent earnings call:

Consider that during this peak we averaged over 30 million deliveries per day. Looking back just two short years ago, that's about 6 million more per day than we handled in 2012. In those two years, we saw peak volumes go from 55% above our average day to 75%.

Now, consider the process by which UPS management calibrates how it deals with peak demand. Essentially, it has only had a few data points to deal with. In other words, a few days' worth during peak demand periods in the last few years. In this context, it's not that surprising UPS has had difficulties.

However, UPS is learning from the process, and is taking steps to increase productivity and smooth out shipping volumes during the holiday season. Plans to implement peak residential surcharges are in place. UPS is adding permanent hub capacity (to avoid incurring the extra expense of temporary hiring and training), and the "control tower" (how UPS manages the flow of shipping volumes with its large customers) will be increasingly focused on yield growth.

Finally, consider that UPS' chief rival, FedEx (NYSE:FDX), reaffirmed its full-year guidance in a press release on Jan. 23. That FedEx doesn't appear to have faced the same magnitude of problems in dealing with peak demand suggests this is not an insurmountable issue for UPS.

E-commerce growth is also a tailwind
Second, amid the doom and gloom of the fourth-quarter results, it's easy to forget that e-commerce growth is actually a great opportunity for logistics companies. Indeed, UPS management expects U.S. domestic package volume to increase by 4% in 2015. Moreover, analysts have UPS growing overall revenue by 3.7% and 5.3% in the next two years, respectively, while FedEx's revenue is forecast to increase by 4.7% and 5.5% in its two next fiscal years to mid-May.

Meanwhile, FedEx predicts U.S. GDP growth will be 3.2% and 3.1% in the next two years. It's clear these two companies -- whose top-line growth rate is traditionally associated with U.S. and global GDP growth -- are finding it easier to outpace GDP growth, partly thanks to growing e-commerce demand.

US Change in E-Commerce Sales data by YCharts.

Business demand growing
Third, UPS could see some margin improvement next year from an increase in business-to-business, or B2B, shipments. "B2B business was up over 3% in the fourth quarter," CFO Kurt Kuehn said on the earnings call.

Given the B2B growth in the fourth quarter and Chief Commercial Officer Alan Gershenhorn's disclosure that management saw "a revitalization of growth outside of the B2C [business-to-consumer] retail market. Five of our top six industries are growing now," it's reasonable to conclude that B2B -- which tends to be less peaky than B2C -- will contribute more to profits in 2015.

Where next for UPS?
Underlying growth conditions from e-commerce and rising B2B demand look good for UPS, and management hopefully has a better handle on how to deal with peak demand in 2015. Industries in transition sometimes suffer problems, as managements wrestle to get a grip on a changing environment. However, the "problem" facing UPS is one of better execution; it's not one of lacking end-market demand. A "problem" that many other companies wish they could have in a period of moderate global growth.