Alignment for a grand tour will occur next in about 175 years as a consequnce of the orbital periods of the four outer planets. The dominant factor is the ~165 year orbital period of Neptune, which sets the "low frequency" interval that defines the overall period. The long orbital period of Uranus also plays heavily, but the fact that its period is 84 years (or almost exactly half that of Neptune) helps keep a lot of things "resonant" in the sense that in 168 years or so, Uranus comes back to the same spot where it was and in that amount of time, Neptune is also in almost exactly the same place. Saturn's orbital period is between 29 and 30 years, and Jupiter's is about 12 years, so they return to the same places relative to the sun more frequently, so their effect on stretching the "grand tour" period is a bit lower. But consider what might happen if, say, Saturn's orbital year was 70 earth years. It'd be back to where it started in just 70 years (too early for Uranus and Neptune to be aligned), 140 years (also too early for Uranus and Neptune to be aligned), and 210 years (too late to be on the tour). In a way you can think of the phenomenon of alignment a bit like strumming a chord on a guitar. The notes aren't the same on every string, but if they're at the right intervals, you get something that sounds good and you have a kind of resonance. Uranus and Neptune are about an octave apart. Jupiter and Saturn are at higher notes on the staff, but they work, and the period for the tour gets to be something reasonable.
Part of what makes taking a tour like this attractive -- other than not having to travel to one side of the sun to get to one planet and to the other side to get to the next one -- is that gravity assist to change momentum becomes possible. Remember that a momentum vector has both direction and speed, and with a gravitational interaction between probe and planet, the probe can exchange momentum with the planet, thus changing both its direction and speed. (It also, to a miniscule degree changes the direction and speed of the planet as it goes by, since energy is conserved, and the extra energy the probe may pick up has to come from someplace. But you'd have to send a staggering number of probes past a planet before you changed its orbit in any significant manner.)
When you travel past the planet ahead of it, the planet's gravity is behind you, and it tugs on your mass (and your momentum vector, pulling you back and slowing you down). If you come into an orbit on the back side of its direction of travel, your mass (to an infinitesimally small degree) exerts a gravitational pull slowing the planet down, while at the same time, the planet pulls on you, transferring energy to you and slingshotting you forward. You momentum vector then has a change in both speed and direction, and you're off to the next planet that much more quickly.
You can liken this effect to towing one car behind another with a rope. If the car in front has a running engine and the one behind doesn't, the one in front is expending energy to accelerate the one behind. And the one behind is stealing energy from the one in front, which would be getting much better gas mileage if it weren't being used as a tow vehicle.
It's worth mentioning this "momentum exchange via gravity" because it also factors into how the Apollo missions got to the moon. One of my favorite mission patches is the one from Apollo 8, which depicts a figure 8 around the moon. (Apollo 8 was the first mission to go all the way to the moon.) You could just write this off as cool graphics and a play on the number 8, but in fact this is the actual trajectory the spacecraft took. It effectively did its translunar injection (TLI) burn to get into what would have been a very high elliptical orbit around the earth had it not also put the spacecraft in the path of the oncoming moon. Instead, far enough away from earth, the ship was captured by lunar gravity as it went ahead of the moon. And it was this negative exchange of energy to the the moon from Apollo 8 that slowed the ship down by enough that it was captured into orbit. Had Apollo 8 approached from the other side (behind the moon), lunar momentum would have been transferred to the ship, and it would have continued on into deep space. Coming home, the engine burn on the ship got Apollo 8 enough momentum to get out of the lunar gravity well and far enough back into its original high orbit around Earth to be recaptured by the Earth's gravity well, where final engine burns could slow it to atmospheric re-entry speeds.