I had to think about this one for quite some time. It wasn't immediately clear to me why I still have a strong liking towards the series, and the original film.
First and foremost, I must say, I've fallen in love with the characters, and I find it very interesting to watch how they evolve. This maturation is clearly shown in the original film, from hatching and being taught the segregation is a fact of life, to working together with all sorts of dinosaurs to accomplish a common goal.
There is a wide variety of emotions found in the plot, none of which seemed forced into the film or onto the audience.
Taking a look at the movie, we see a natural flow of emotion, starting with the meeting of new friends (Littlefoot and Cera), and the different attitudes regarding being friends, to the instant common ground they share when Sharptooth arrives to do them in. They are instantly forced to work together to survive.
There is then the feelings of loss and sorrow when Littlefoot's mother is killed. A renewed feeling of life is brought on by the meeting of Cera and later Ducky. As the gang works together more and more, elements of a certain kind of love emerge between them, to protect each other and to make it through, together. The joy at the end of the film is a representation of this success.
The show of emotions in this film is truly amazing.
Of course I need to mention this

but the music found throughout the film is fantastic. From the opening titles, the music captivates the audience similar to the effect of music on some Disney classics as Fantasia. James Horner (the composer of the Land Before Time musical score) an incredible job in bringing the film to life. I must admit, I'm not sure I would have loved the film so much if there wasn't the masterful work of Horner present. The music tends to carry the film nearly as much as the dialogue, and I respect that. The music also enhances the emotions of many scenes. For example, the relaxed, yet oddly uneasy feeling when Littlefoot and Ducky are walking shortly after they meet, or pure, warm feelin when Littllefoot and his mother see each other for the first time. Just amazing work all around! B)
As for what keeps me coming back, as opposed to other very good animated films, it is a combination of the unique characters and the fact that it is part of a long-running series. The existance of the LBT series certainly helped hold my interest, as the other films help add depth to the characters, places, and stories. I always found it interesting to see what else the storytellers would come up with.
I still find that LBT 2 is my most-watched sequel, not because it's been around the longest (well, that too

) but because it seems to have a very strong connection to the original film and its character styles. It felt like an extension of the original in a way. The later sequels almost seem to drift into their own series in my mind. Anyways, I'd better stop, as I'm getting a little off-topic!