Once again, the Dow Jones Industrials (DJINDICES:^DJI) has vaulted above the 14,000 level, coming back from the long holiday weekend with some optimism about the market's future prospects. Major news sources are pointing to renewed expectations for more merger and acquisition activity as the reason for the advance, as low interest rates and income-hungry bond investors make it easier than ever for companies to get the financing they need to make buyout deals. By 10:45 a.m. EST, the Dow was up 47 points to 14,028, and broader market measures were up almost half a percent.

Nowhere is the M&A trend more obvious than in the office retail space, where Office Depot (NASDAQ:ODP) and OfficeMax soared 14% and 27%, respectively, following reports that the two companies are in talks to merge. Surprisingly, Staples (NASDAQ:SPLS), which arguably has the most to lose if its rivals combine forces, is also up in the double digits this morning. The potential positives and cost-savings of an Office Depot-OfficeMax merger are obvious, but Staples also stands to benefit if the move results in a less cutthroat competitive environment that allows the survivors to make their business models more efficient.

UnitedHealth (NYSE:UNH), on the other hand, has fallen 2.4% since Humana announced that the federal government's proposed Medicare Advantage payment rates for 2014 would hurt profit margins substantially for the segment. If those rates become final, UnitedHealth may have to scale back its private-insurance offerings to Medicare recipients, which represent a significant part of the insurance carrier's business and could have a material impact on revenue.

Finally, Melco Crown (NASDAQ:MPEL) fell nearly 7% as analyst firm Sterne Agee said that growth in gross gaming revenue in Macau could rise much less than expected this month, with a traditional cooling-off period following Chinese New Year celebrations holding back the market. Even more so than U.S.-based gaming companies that have expanded into Asia, Melco is vulnerable to threats of a slowdown in Macau due to its near-complete focus on region.

Fool contributor Dan Caplinger has no position in any stocks mentioned. You can follow him on Twitter @DanCaplinger. The Motley Fool recommends UnitedHealth Group. The Motley Fool owns shares of Staples. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.