Behaalotecha - Masaot

The Divrei Kehillos pg 89 and pg 104 mentions that we sing the maso'os in a happy melody (and see there in page 450 that the V'al Tzevo in Beha'alouscho have the same melody), but he doesn't explain the reason for it. Harav Yonah Merzbach explains that since Rav Ashi (in Avoudoh Zoroh כד Omud ב) says that in every one of the maso'os, when Moshe would say Kumah Hashem Veyofutzu etc., Bnei Yisro'el would answer with the song "Roni Roni Hashitoh" (The Oron shall be happy) etc., see in the Gemoroh there.
Therefore, since all the maso'os were done happily and with singing, we sing the maso'os when we read them.
[I'd like to note that Harav Wolf explains it in the opposite way - he says that the melody of the maso'os is similar to the melody of Eichoh(!), and it is done because Am Yisro'el had later on many maso'os in the diaspora].

Michael FRBSH

The Maso'ous on Parshas Masei are read like Parshas Behalousecho as mentioned in Divrei Kehillos pg 89. He mentions there that the longer psukim are read in the regular ta'amim.

Michael

For the reason given above, that every maso was done while singing. The Divrei Kehillos pg 89 adds that the reason for singing the maso'ous is as explained in Rashi (at the beginning of the Parsha) - and most likely he's referring to the second explanation in Rashi, that when a boy was sick and got better, he enjoys looking back at all the way that he took when he was on the way to recover.
It seems from Divrei Kehillos (pg 104) that the reason only the short psukim are said in a melody is practical, since that's the way it can be sung properly.

Michael

In KAYJ we do break the maso'ous and on Mincho of Shabbos Matous we read the regular laining brought in all siddurim. On Shabbos Massei we started the second Aliyoh one posuk earlier (Vayis'u me'eilim), and the Divrei Kehillos (pg 104) brings that the Noheg Katzoun Yousef understood that the reason for this was in order not to split the maso'ous, but he argues with him saying the maso'ous start much before therefore this is not the reason we do so.
The reason the Divrei Kehillos gives for starting the aliyoh one posuk earlier is very interesting - he says it is done in order that the second aliyoh will not start immediately with the maso'ous nigun, since it's much more pleasant to the ear to start with a simple melody and then change to the happy (maso'ous) melody! If this is the reason - you can see how important the Maso'ous melody was considered, that the aliyoh would start a posuk earlier in order that we get the complete feeling of simcho from the maso'ous melody!

Michael