Learning a new language is an undertaking. There are so many words to pick up and grammatical structures to get accustomed to. Then there are sounds you're probably not used to making.
One of the factors that determines the difficulty of learning a new language is the languages a person already speaks. Babbel shared the four easiest languages for English speakers to learn:
Dutch is spoken throughout the Netherlands and by a large number of people who live in nearby Belgium. After German and English, it is the third most widely spoken Germanic-based language. That similar Germanic base to English is part of what makes it so easy for English speakers to learn, especially since its grammar rules and audible tones have more in common with English than German. In fact, if you speak both German and English, Dutch can be mind-boggling to listen to, since it sounds like a perfect combination of the two.
Cognates, which are words that look and/or sound the same in two different languages, are also a determining factor in how easy it will be to learn a language. For English speakers, Dutch is full of these. While you will occasionally encounter a false cognate (a word that looks the same but is actually different), if you're vigilant in your learning, Dutch is easier than most.
Since Spanish is such a widely spoken language in the United States, it's much easier for most English-speakers to find what linguists call authentic input, that is, written or spoken media in the target language. Most people have at least one Spanish-language channel on their local TV or radio listings.
To go with the authentic input available to you, Spanish is also a romance language that, like English, derives from Latin, making it easier for English speakers to pick up elements. And when it comes to word order and pronunciation, Spanish is fairly easy to understand, making it far less difficult to master the grammar and sounds. Given all these advantages, there's little reason not to learn the second-most widely spoken language in the world.
As with both Dutch and Spanish, the key to Swedish being a relatively easy language for English speakers to learn is cognates — Swedish is filled with English cognates. In addition, Swedish isn't nearly as fussy about word order as some other Germanic languages, which can be a massive hurdle when trying to decipher sentences.
There's also one Swedish corporation that has unintentionally made it easier for English speakers to pick up Swedish — IKEA! Whether you realized it at the time or not, simply shopping for furniture at the mega-store exposes people to the Swedish words for a lot of everyday objects, so a trip to the store can be an easy way to practice some basic authentic input.
Much like with Swedish and Dutch, the main reason that Norwegian is easier for English speakers to pick up is the Germanic base it shares with English. This leads to a number of English cognates. While you will encounter different spellings, of course, many are easy for English speakers to parse — it doesn't take much thinking for an English speaker to figure out the meaning of Norwegian words like vinter and sommer, for instance.
There's also only one verb tense, which significantly reduces the difficulty in learning how to form grammatically correct sentences. Norwegian isn't considered an especially wordy language, and that economical use of words means you're closer to forming meaningful sentences from the get-go. Compared to picking up other languages, learning Norwegian is a piece of cake.